I think you are grossly overestimating the effect of this change. No one other than Dredge is going to mulligan to oblivion to find one card as you still lose a card every time you mulligan. The new rule just grants you potentially better card quality on your average card in hand than your opponent instead of a strict disadvantage of down cards AND same card quality on your average card.
This argument could be made about any rule change with this kind of impact. For that reason, I find it very >unconvincing.
No mana burn, no damage on the stack, and other rules changes that dumbed down the game still feel very un-Vintage to me so there's that.