I’m really hoping that Valve will end up being a better stewart of the PC ecosystem than Microsoft in the long run.

Outside companies can't really contribute to Windows development and have no say, so it's 100% up to Microsoft to look after the Windows ecosystem.

On Linux however, Steam can contribute to the kernel and both audio and graphics systems. All this apart from delivering their Steam service. The same will be the case for GOG, although they probably can't afford much, and the same will be true for Epic if they want.

Linux has mainly been held up by server development on the professional side, and desktops have been mostly been an afterthought among the ones with the money. Even Linus Torvalds does not use Linux as his personal computer. So most of the Linux desktop was developed by personal private interests with help from Red Hat.
But despite that Linux desktop is pretty damned awesome today with strong alternatives to Gnome. 😀

It can never be the responsibility of Valve alone to keep the Linux desktop alive and thriving, they are merely part of it, and they have been a great boon for Linux, like Red Hat used to be who developed most of the GTK graphics back end, Gnome desktop, Pulse Audio and systemd.

Linux desktop is a very very different ecosystem than Windows, in the way it works, kind of like how Eric Raymond tried to describe in his book The Cathedral and the Bazaar. Unfortunately he failed, and the book and ended out as mostly nonsense, but the book is famous anyway, because it does try to describe the gist of it.

Sometimes, It does really feel like the PC ecosystem can wither and die

The traditional PC is dying, no doubt about it. PC used to be a universal tool for everybody, it isn't anymore, many people have scrapped their PC, and now just use their phones and sometimes also tablets. The PC is a special interest tool today, and as such it will survive, but at a smaller scale than it used to be on. But as a special tool, it will also keep developing for those special "needs" like gaming, and the PC remains way ahead of any other form of game system in almost every way.

Edit:
Linus Torvalds does indeed use Linux desktop today, he Apparently uses Fedora.
But for many years he didn't.