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.