Depending on the landscape, it will not be up to Microsoft, though. If enough SteamOS machines are on the market, it will be a viable target platform on its own, and then, even if MS had a special part of the API, the developers won't use it, lest they lose the SteamOS market.
Microsoft has many ways to tackle the problem, starting by the amount of studios they could take out of Steam moving exclusives to their store, console style, as one of the biggest publishers.
Then there are enough shennigangs they might think of regarding APIs, legal actions against Proton, or whatever their creating minds can conjure.
There are low hanging, unpicked fruit for Microsoft to make more money from selling desktop licences. It's still trivial to buy one for a third party for pennies on the dollar. It's still not possible to install officially without TPM 2.0.
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.
I'm sure there's a lot they could do, but I think even is the worst scenarios, SteamOS would still be a vibrant indie platform, with many major releases from studios who dgaf about Microsoft specifically.
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.
I'm not worried. Strong API backwards compatibility is one of (if not the greatest) Windows moat. Microsoft risks their market dominance if they begin fucking with that. Especially with regards to business use cases.
Not really, otherwise Microsoft wouldn't keep pushing WinAppSDK and WinUI, however I do agree it doesn't get much love, after all the mess, including not taking backwards compatibility into account every single time they rebooted the developer experience since Windows 8.
In case you missed the memo, WinRT last reboot was to make it work on Win32 side, and more recent COM APIs are mostly WinRT variants.
This will be like the netbook wave, or OS/2 Windows compatibility layer, a celebration until Microsoft decides the show has had its time.
Valve really should push for native Steam OS builds.