But they do test their Windows games on Linux now and fix issues as needed. I read that CDProjekt does that, at least.

How many game studios were bothering with native Linux clients before Proton became known?

That goes back to address the original question of "But would you want to run these Win32 software on Linux for daily use?"

Maybe Valve can play the reverse switcheroo out of Microsoft's playbook and, once enough people are on Linux, force the developers' hand by not supporting Proton anymore.