I think the time to do that has already passed; the Steam Deck and even Linux gaming in general has really taken off, so it would impact far more people than before. In addition, I think the EU has started breathing down their neck and will pounce on them if they do anything so blatantly monopolistic.
So this hypothetical “Direct X 15” would be limited to only Windows 11 users outside of europe, and the games wouldn’t be distributable through Steam. I can’t see that being a good pitch to developers, even if DirectX15 was really good.
They also tried this waay back in Windows 8 with the Windows Store, and it honestly was a flop.