One thing that gets me with proton is that as you say it effectively 'crowns' win32 as the PC gaming platform, but it seems like a weird situation where MS control it and valve/codeweavers are constantly chasing them for any new/changed functionality so their sub-platform remains relevant. I think an opportunity has passed to divorce PC gaming (or perhaps "consumer real time 3D"?) from MS/windows because Valve don't want to take on all that responsibility, and no one else is interested enough to set up a consortium to pick up that gauntlet.
This is my cynical side, but I'm sure they don't mind the opportunity to get their store in front of people, both with the deck and by how closely knit steam is to providing gaming to non-windows PCs. In my view PC gaming is in a weird spot right now if you try defining "what is the platform?" Is it windows, is it steam (and all the other features it has), is it x86, how much can/should a game be 'portable' from one ecosystem or enclave of PC. There's also been issues with games like Starfield not working with the intel Arc GPUs until a few days ago that have me wondering (from a fairly naive point of view) how closely that aspect is tied to assuming nvidia/AMD are the only possibilities versus how well it was written to the abstraction layer, assuming intel were compliant.
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 :)
This is my cynical side, but I'm sure they don't mind the opportunity to get their store in front of people, both with the deck and by how closely knit steam is to providing gaming to non-windows PCs. In my view PC gaming is in a weird spot right now if you try defining "what is the platform?" Is it windows, is it steam (and all the other features it has), is it x86, how much can/should a game be 'portable' from one ecosystem or enclave of PC. There's also been issues with games like Starfield not working with the intel Arc GPUs until a few days ago that have me wondering (from a fairly naive point of view) how closely that aspect is tied to assuming nvidia/AMD are the only possibilities versus how well it was written to the abstraction layer, assuming intel were compliant.