Framing the discussion as 5 Mox vs 0 Mox seems unnecessarily extreme to me. Cantrips are great, and cutting mana for cantrips is a tried and true option. Don't think of things in terms of "4 Preordains means 0 Mox" ... figure out how many mana sources you want, then figure out how many of them NEED to be on-color, and the rest are your Moxes ... maybe that leaves you with 3 or 4. I don't think there's any reason you can't build a functional Oath deck with 3 or 4 Mox. Vintage is pretty slow right now, a turn 2 Oath is still a good play a lot of the time.
In and of itself, Dack Fayden is a great card, and non-creature advantage-generating permanents like Planeswalkers seem good to me in Oath in general.
Still, I think there's some things you might not be considering.
Gush and "The Best Draw Engine in Vintage"
The reason Delver beat other blue decks wasn't just the low mana count, but the strong synergies between Gush, Dack Fayden, and Delve spells. Delver ran such a low Mox count because it needed to keep its Island count high enough to support 4 Gush.
Without a steady stream of Gush and Delve spells, Dack Fayden doesn't put you ahead on cards same way it did before.
So there are a few problems here. Of course many these cards are restricted now, making the whole package worse, but many decks still run them. Gush is problematic for Oath because of the Forbidden Orchards, though some lists run it anyway.
Oath has a unique problem with spells that draw a bunch of cards - getting stuck with creatures in hand. This is why you'll see a lot of Oath decks with Dig through Time but no Treasure Cruise ... and without Treasure Cruise, Dack Fayden gets a little worse.
If you're going to be running Dack Faydens, I would imagine it's necessary to run some way of mitigating the damage caused by Dack'ing into creatures. Maybe use something that shuffles, like Emrakul, the Aeons Torn? Personally that's one of my least favorite Oath targets, but it might be necessary.
Note that the -2 ability on Dack Fayden is just a little worse than it is in a Mentor deck, which leads me to ....
Where are you trying to position yourself?
Let's say you could cut mana and make your deck better against blue. That might be true. Is this something you want? The reason Oath Control has made its way to the Decks to Beat page isn't because it's dodging blue decks. (And make no mistake, while there are dozens of Oath variants, only one has really proven itself tier 1 in the metagame right now). Anything involving more Preordains is a worthwhile experiment as far as I'm concerned, but I would be REAL careful that you don't inadvertently kill the Workshop matchup while you're at it.
Still ... It's a worthwhile idea, there's just tradeoffs. Keep in mind why this isn't normally done, and what else you'll have to change to do it.