If enough players play via Proton and want to stay that way, then the subset of the Win32 API that is well supported for Proton can itself become the standard.

If enough players play via Proton and want to stay that way, then the subset of the Win32 API that is well supported for Proton can itself become the standard. MS might add new APIs, but developers may choose to keep the old ones, therefore targeting the largest userbase. Of course, MS might also start changing the behaviour of lots of APIs, deprecating them left and right, effectively trying to kill Proton. But that means they'll also kill their own backwards compatibility. If, again, Proton becomes "popular enough", I don't see how MS can stop it short of finding some way of preventing developers from using old APIs in new applications (which would very likely be anti-competitive behaviour). I've been a macOS user for 11 years now, but I hope that, however unlikely, Proton wins out :)