I'm entirely sure they'd be happy with you re-purchasing your games through the MS store.
The bigger problem with the backlog of games (and other software entirely unrelated to games) is that Steam people aren't stupid or lazy, but many other developers are. Short of literally locking the platform (which would garner them some serious anti-competition lawsuits,) at the end of the day they have to allow software to run on their platform. Nobody uses an OS purely for the file explorer. Third party apps are a necessity.
Which means Steam can just continually upgrade their app to match. They're doing that regularly anyway to keep up with video driver changes and other tweaks.. its a bit of a pain but having one more source of reasons to push patches isn't exactly going to break them.
What would get broken is everybody else who doesn't have the money or manpower to keep up with a constant stream of breaking changes. Never mind the developers that have gone out of business and end users that rely on their products go from unsupported to completely SOL.
MS really can't "break" Windows in any significant way. Sure they might try to tweak or deprecate specific APIs that Steam happens to use and few other software packages do, but I'm sure the Steam programmers have the capacity to work around things like that. And the main Win32 API can't just be pulled without driving off a large portion of your customer base that now can no longer use all of the (non-game-related) apps they rely on. Especially corporate customers who make up a large portion of MS' income and give exactly zero shits about Steam or games. Probably prefer if their users aren't able to play games on their work systems. But they sure as hell care if their $50,000/seat CAD software stops working every few months.