What is interesting is we keep looking at decks that benefit from the mulligan rule and try to figure out if any of them will break it. I think realistically we are forgetting 2 things here:
1 - A deck that naturally wants to mulligans fewer time than it's opponents overall will still be in better position to win most games. It would still be better to play a deck that has good ratios in its decks that lead to naturally stronger opening hands on average. In this regard "fair decks" like Humans/hatebears might actually be the big winners with this new rule. Being able to keep an opening grip of 7 that has like 3 pieces of relevant hate and all the mana I will likely need is stronger than mulling to 4 for a single card that may be answered.
2 - Decks that specifically do not roll over and die to one magic bullet are very well positioned in the new mull rule because they not only benefit from having more consistency with the new mull rule, but they also do not have to worry about their opponent being able to abuse the new rule to hose them out. Think something like a control deck that has pyromancer as a win con, but also has blightsteel tinker vault key backed by counter magic. In a lot of ways this rule is going to help blue more than others because free counters will be at more of a premium to stop hate cards.
If they are not willing to restrict Workshops and Bazaar why would you think they would ever ban cards that are limiting to the format like Lotus of Moxen? And we know they will not reprint them for legal reasons.
You may not like the idea of 3 of or singleton, but it is at least an option at this point where are where as the 2 you have laid out have been long dismissed by WOTC.
I think you also have to consider the following:
Is playing a non zero number of powders that isn't 4 potentially correct? The first one seems to give more of a percentage boost than the subsequent ones so it could be that 1 is the correct number, or even 2 or 3.
For games 2-3, do you side the powders out for more hate/anti-hate, as putting another single piece of hate in your list could increase your percentages for an opening hand more than the 4 powders. Do you side out all 4 or is there another more optimal set up. Perhaps having 1 in hand in the course of a game comes up enough that it makes sense not have more than 2 post board or something like that.
If you are Mulligan and powdering more frequently because of the rules change does that change the proportion of card types you'll need to build with to ensure you don't powder away to many, or perhaps because you can now "save" cards from powder does it open up what you can include.
I think that every restriction has its own wrinkle to it, and that some of them function as a psudeo ban and some of them are true restrictions. I also think that card use from the restricted list will likely ebb and flow with changes but overall trend upwards over time.
As for cards being restricted for intrinsic properties vs Metagame, I'm not sure you can say it is either or both. Gush has been on and off the list multiple times. Was that a meta game adjustment or is the card inherently just that damn good? Is chalice somehow more good now that it was before, or was it's restriction just an oversight/overdue?
WOTC is clearly reacting and adjusting to the state of things as they are with most restrictions, as opposed to trying to be predictive and cut off issues before they come up, so I believe the metagame metric is what they are most leaning on, but they have flat out said that this is not a purely data driven descision as they are trying to not affect the "pillars" as it were. I just happen to fundamentally disagree with the concept of a pillar being out of bounds for restriction, as a pillar can be restricted and still be viable, as dredge and shops are more the sum of their parts than any one card.
Paper Vintage is a doomed format because of the Reserved List and the price of the cards. This establishes a barrier to both entry and exit with a resultant inertia in the player base. When the cards are accessible, you have an influx of players who enjoy the format and an efflux of players who don't enjoy the format. Even if the format hypothetically becomes several decks of 75 restricted cards, you will still have players that enjoy the format. It would be EDH (which isn't exactly a fringe format) with Power and I'm sure Brian Kelly and I would love to pit Dromoka vs. Silumgar in an epic sh0wdown. Thinking that a partial or completely Highlander format is unviable ignores reality.
I would 100% be an advocate of a singleton vintage format, don't get me wrong. I think it would inherently be far more balanced than what we have now quite frankly, but it would be a radically different format. That is my point though, that some variant of the second law of thermodynamics seems to apply here in that the format will forever trend towards a state of decay until it by design or by default is something unrecognizable to what it is now.
By having more room for sideboard, there is more opportunity for innovative transformation strategies, and more space to pack specific answers to threats. These things should ultimately leads to more skillful games.
Vintage is a doomed format if you draw the time line out long enough. Where as something like standard that rotates can reuse cards ad nasuem and create new, unique pools to pull from all the time, there will come a point in vintage where the restricted list will no longer work to balance the format because we will hit a critical mass of mistake cards.
It has been speculated that eventually most decks could be 75 unique restricted cards, because that is how good some of the cards will be. The printing of treasure cruise, DDT, and Monastery mentor have shown that WOTC makes mistakes / Does not power check against vintage, so we can expect more of them with some regularity.
Using TC as an example, there was a time where having 1 ancestral recall in the format was the standard. They are unique cards to be sure, with pros and cons for both, but the reality is you now have 2 physical copies of a draw 3 card for U and it fundamentally changed the format, even with a restriction. How long before we get another time walk variant that proves a bit too good, or an innocuous cantrip that suddenly interacts with something only vintage players use. Wotc is intent on printing Mox variants every few years, so it may not be long until we see another one that is on par with something like Opal.
So at some point in the future, and I do not know when that would be, the format will degenerate enough that just the restricted list will not cover it and you have to start taking radical actions. Banning is the simplest one, but that often just serves to preserve the status quo because they typically only ban the new cards. Increasing deck size or reducing max copies becomes another option just to force more variance into decks to reduce the occurrence of broken plays, which is the opposite of what the new mulligan rule is attempting to do.
My overall fear is that the London rule reduces variance too much. I don't think that decks should have that kind of starting hand consistency. I wouldn't mind it as much if starting deck size were a bit larger, but I think it makes it too easy to find a single card, and also to assemble 2-card combos.
Mechanically this is problematic for a number of reason. Larger decks in paper lead to longer shuffle cycles and are more cumbersome. I'm not saying 70 is substantially different than 60, but it depends where you set the limit.
Increasing deck size would increase variance yet again, which is counter to what they are trying to achieve. I don't think I need to explain the math to you but a 120 card deck that is literally just 2x of every thing in a 60 card deck will have wildly different hands because of how the odds change every time you draw a card.
If I draw a card from a 60 card deck with 30 lands, I have a 50% chance that my first card is a land, then if I drew a land a 49.1525% chance of another on my second draw from it. If I drew a non-land my chances of a land are now 50.8474%.
That same example, with 120 card deck with 60 lands, is 50% on the first card, 49.5798% on the second if I drew a land, and 50.4201% if I did not. The gamut between the numbers gets wider the more you go down the hyper geometric wormhole. I know the difference does not seem substantial but in the long run it does lead to a swingier range of possibilities. It also has the consequence of making tutors more powerful since they have a wider range of options, mill less viable (not really an issue but still) and cantrips better in some ways and worse in others.
That being said, if you want to increase variance without distorting proportions or increasing mechanical challenges, you could go to a 3 of a kind format. This does have the added bonus of limiting cards that cause issues when they compound upon themselves like spheres, as well as reduces the price point of the format. It still has some other consequences like making tutors more powerful by increasing the range of cards they inherently can find, but you can already build your deck that way if you so chose.
Also, totally just speculation, but what If Wotc uses the Modern Horizion set or some other off standard set to print much worse legendary versions of workshops and bazaar and then restricts them in vintage. Would that soften the blow to people if they made a legendary workshop that only made 2 mana and a legendary Bazaar that was draw 1 discard 2?
Maybe not viable because of Modern but still.
I have been brewing like crazy lately and the format is much more open than you would believe. If you just play main deck Leyline of the Void or explore Cavern of Souls, you will see that there is room for innovation.
So the format is wide open so long as you play along these narrow lines?
You want an open format look at Modern. Phoenix is in a bit of a high spot right now but look back at it pre-phoenix and that format literally has 10's if not 100's of viable builds that do not have overlapping cards. There are entire lists like living end that use cards that no other deck would even dream of, and virtually any known archetype has a viable list. These decks are not variations on a theme, its not Jund and then Jund splash blue, they are totally unique lists. The fact that the format is so open actually breeds more viable lists because you cannot fine tune your lists against the meta with specific cards like we do in vintage.
Vintage has the largest card pool in all of magic and one of the narrowest lists of viable decks available. Even decks that are considered spoiler/fun police lists in every other format (burn) are not even viable in this format because they cannot utilize the few most powerful cards in the game. I'm not saying that every deck has to be viable tier 1-2 lists but virtually any list that isn't established right now and cannot run some sort of power is barely fun tier and some are so non viable they are not fun for either player. Many decks get pushed out of being even remotely playable because of the splash hate for other lists.
Restrictions fix that, they have historically been used to fix that, and should be used. Right now there is a small oligopoly of tier one decks that push 99% of the viable decks and cards in the format right out of contention, and that is a problem.
Personally not a huge fan of playing meta decks, and workshops is still too much concentration of wealth for my tastes. Playing something that can win any match, is not specifically blown out by anything, and is something that gains edges from new releases interests me.
Were shops restricted I would pour my attention into some sort of eldrazi prison deck, because I like the idea of intentionally playing something that wants no power and no power lands and does things that are very strange when you consider the format, but shops eats that decks space right now so moon is way more interesting to me.