The issues to me isn't about tunning games as i think they could di that, and game companies would have a reason ti make sure it works smoothly with steam OS. But i think for many the main issue would be how compatible is everything else you use. People love linex but look at the 1ks of things that don't just work and need lots of tweaking or flat out wont work, that's one reason linex isn't more popular. Heck even as big/popular as apple and Mac are there is tons that won't work on Mac OS. It's one reason I've never bothered with one. Sure I could deal boot now, but what's the point of a Mac when I got to run windows anyway?
People could have a gamer rig, but so much is about everything else, that would be the real test, outside of security.
In the end people love to hate on windows, but in the grand scheme of things, I've never really had an issue with them out side of getting a few games to run on modern systems. I've built every system since my p166. It's a fun hobby in itself. Proper set up and quality parts I've had very few issues with any win os. The only one I haven't tried yet is xp and 11. Still I'm open to other options, but in the end windows is pretty cheap and it works. 300 bucks in 25 years isn't bad