Steam Machine

Very interesting! The one killer issue that jumps to mind is anti-cheat. I switched away from gaming on Linux via Proton to gaming on Windows because Battlefield 6's anti-cheat won't work under Proton. Many games are like this, particularly some of the most popular (Rainbow 6 Siege for instance). And BF6 made this decision only recently despite the growing number of Steam Deck players (and other players on linux - in fairness I don't think there would have been that many BF6 players on a handheld).

Edit: I specifically use a gaming-only PC. The hardware is used for nothing else. Hence, discussions of rootkits don't really bother me personally much and on balance I'd really rather see fewer cheaters in my games. I think it would be the same with any of these machines - anything Steam-branded is likely to be a 99% gaming machine and their users will only care that their games work, not about the mechanisms of the anti-cheat software.

All Valve has to do is say “Your software cannot deliberately exclude linux support including kernel anti-cheat to be listed on Steam.” And that would be that, the few devs big enough to make it on their own would leave, and everyone else would adapt.