"Will it play GTA6, and play it well, whenever that eventually comes out?"

That is the bar (in my opinion) today, you have to take your box over to rockstar and spec for that or you are just selling outdated hardware.

I assume people who choose a Steam Machine over PS5 know what they are doing.

On the other hand, people are probably dumber than I think.

For what it's worth, Rocket League plays just fine on Linux if you use Proton, but your point on the others still stands.

I've had to stop playing a few games once I made the switch-over (Destiny, GTA V), but am otherwise very happy with where SteamOS/ Proton is.

There are presumably mobile games which have even bigger playerbases. Most of the "regular gamers" of this sort I know will probably not buy a console either. Does it matter?

It really says most about what people you hang out with.