For them DirectX and Win32 is what matters, if folks go out of their way to run on Proton, that is Valve's problem.
Literally half the gaming/hardware focused channels I watch have run at least one, if not several Linux Gaming videos and tests this past year... mostly in the past 4 months and mostly praising the state of Linux gaming. It's not going away.
I don't think so. I rather do believe that many game developers would actually love to give a more native approach for writing GNU/Linux games a try (to make this point more plausible: game developers are very used to game-console-native SDKs).
But what these game developers really demand is a very stable user-space API for everything that is necessary for writing games, which will work reliably on basically every GNU/Linux distribution, and will be supported for at least 20 years.
And studios definitely check out their games running on Steam Decks via Proton now, so that's good.