Until the fountain runs dry, because the kingdom lords diverted the river from Valve's well castle.
Valve is pushing mostly open source to expand to other platforms which is a win win for everybody.
Thousands of game studios are gone now, and supporting their software is important legacy work. You don't have to appreciate that, but I do. I do not give the faintest fuck about the opportunity cost you bemoan towards native UNIX games when I do this. That's your problem, not mine.
If Proton is "Linux" games, so is running GNU/Linux under a VM a proper distro.