I wish things were working so seamlessly for me, as people describe in the comments. There seems to be something wrong with Steam and how it works, so that in my machine (and CPU and GPU from 2019, with official Linux drivers from standard repos, running Debian KDE) it almost never manages to start a Windows game. I will click the green "Play" button, it will change to a blue "Stop" button, as if the application was running, then shortly after silently switches back to the green Play button again, without any visible error and without actually starting the game. This has been going on for years and I have tried various things, Including HWE kernel, OS reinstall from Linux Mint to Debian, installing the steam client via various means, and whatnot.

I have a suspicion, that somehow Steam has issues when Guix is installed, which I am always using, but then the question is, why Steam is incapable of just shipping with whatever it needs and using the things it shipped with properly, instead of getting confused by Guix, which only puts things in the GNU store, and not in a place that Steam should ever look at. But like I said, it is only a hunch or suspicion, and I need Guix more than Steam on Linux.

Then there are games that just work, like Stardew Valley. And maybe Terraria. I suspect, that it is somehow also about what engine the games use and what those engines rely on. But these games are very few, and most bigger mainstream games like AoE2 simple won't start, like I described.

So for me it still seems, that it is not actually working that reliably on just any GNU/Linux system, and that there are still blind spots, that Valve or whoever is clearly not seeing or considering in their whole Proton development or how Proton is used by Steam. Probably some isolation thing that they are completely missing for several years now.