If the API only has additions, then Microsoft would still need to convince game devs to actually use them (and Valve will point out that if they do, their game will not work on Steam Deck, so there's a clear downside).
If some APIs are removed, it breaks older Windows games. I can't think of any historical API that has been completely removed in this way - even stuff like DirectDraw and DirectPlay is still there even though it has been deprecated for decades.
They could create a new interface that's somehow more efficient, and work with Unreal / Unity / Godot and a few others so it's just a recompile for them, but it's a bigger problem for Wine, perhaps? I'm just thinking out loud.
They “could” also create a new interface that's somehow more efficient for Windows. Oh wait..
I don’t think MS has the attention span for stuff like that. Especially considering the limited short to medium term payoff.
They could buy Unity though. Considering how mismanaged that company is it wouldn’t be such a bad outcome. Of course large acquisitions are very costly and risky these days.
See ABK deal, Microsoft has tolerated SteamDeck thus far, they own the platform.