My suspicion is, at least in Vintage, that this rule would make mulliganing your average 7 correct. If you have 1-2 more lands than you might want or whatever that you’d probably keep now, this new rule encourages you to ship it instead. I don’t think that’s good.
I don't think that's necessarily true. The impact might be muted somewhat due to the overall higher power level of the average card in Vintage compared to other formats, but I still think going down a card is a big enough drawback that it'll continue to be correct to keep marginal hands like the one you describe.