StingingVelvet: And Microsoft can't make Windows a closed platform (as in requiring their approval to make software for it) and maintain their OS dominance, so I really don't see Steam being threatened that way.
It doesn't have to be black and white, like that either the future of Windows is completely open to everyone and all competition, or that Microsoft completely locks it up overnight, iOS-style. There are at least fifty shades of grey in between.
What we have seen so far is MS nudging the Windows market towards the Windows Store. Step by step, make it a bit more and more cumbersome not to use the Windows Store but still trying to get your games and applications from other stores or elsewhere, hide the options to do so further and further down somewhere that casual users can't find it easily themselves (e.g. to enable "sideloading"), make your own "Windows S (Mode)" devices where it may be even required that you obtain your software from the Windows Store (in which case many people even in "legacy" Windows 10 will also opt for a Windows Store version, just so that they can use the software IF they sometime in the future by a Windows 10 S device where sideloading is not possible, etc.).
We will see what will happen, but based on the linked discussion, to me it seems this is MS's current strategy, slowly nudging the competing stores to obscurity. The possible plans to make Win32 a virtualized "legacy mode" and not offering sideloading in many devices could be pretty big nudges.
Saying that it could never happen to Steam because of their extensive library of (legacy) Win32 games is about as relevant as saying in 1995 that PC game developers will keep making MS-DOS games from here to eternity because there are already so many MS-DOS games/users around. Win32 will probably be the next MS-DOS, first put to life-support, and then finally cut away. Then the main question is, how easy it is for Microsoft's competitors to sell UWP applications, directly competing with Windows Store on an even playfield?
There is a reason why even Valve got a bit anxious and came with SteamOS and invested resources to Linux support, and why GabeN directly showed his dismay towards Windows 8. It seems Valve at least did feel threatened by the arrival of an official Windows Store, they understand what kind of leverage it gives to MS over who gets a cut from software sales on future Windows versions.
StingingVelvet: One thing I know for a fact is that as long as a game has a healthy fanbase someone's going to make sure it runs on a modern OS, and that includes dealing with crazy DRM.
Sometimes yes, but sometimes the fanbase is apparently not big enough, and you still might want to try some old game.
Like, lately I've been reading about the issues of running Black & White 2 on modern PCs. It seems the original retail version has copy protection that might be incompatible with e.g. Windows 10 (and I think also Windows 7 with recent updates), and the existing noCD cracks for the game seem to trigger an internal "anti-piracy" nuisance which makes your population not to age, meaning your village will be full of small kids not suitable for work etc., making the game unplayable. I presume that is some anti-piracy mechanism, similar like some Settlers games having only pigs produced on a cracked version, or some Operation Flashpoint making your weapon accuracy bad or whatever it was.
You might argue that Black & White 2 is not important enough to be preserved, but still there seem to be quite many people complaining about it, people who apparently wanted to play B&W2. I have no idea whether I'd like the game, never played it.
Plus, if and when some unknown guy offers on a torrent site a "100% fix for the aging problem in Black & White 2! Download this small exe file!"... well, better first be sure it doesn't contain malware... I prefer if I don't have to use such unknown fixes from unknown users for my purchased games.