Then there are enough shennigangs they might think of regarding APIs, legal actions against Proton, or whatever their creating minds can conjure.
We'll see for sure, especially if the Gabecube sells well. Right now, SteamOS is still not among the largest players, when looking at units sold. I'm sure Microsoft will ramp something up when it gets more popular.
In contrast, Microsoft have pushed the pricing of Game Pass up significantly and are in the process of unifying the next Xbox platform with PC.
Given that, I don't think it's consistent with Microsoft's current strategy to make selling games to gamers harder for the benefit of the OS division.
Now, that conclusion does depend on Microsoft acting rationally, which isn't a given, so I'll also add that I don't think it's actually an option for them: win32 already exists, the cat is out of the bag. And the cat can't get back in the bag to be extended/extinguished unless Microsoft convinces everyone to move to Windows 11.