If Proton just becomes the "game runtime" for Linux, that's OK! It's still open source. There may come a time when Proton compatibility becomes as important or even more important than Windows compatibility for game developers. Not only might game developers start explicitly supporting Proton, they could even contribute fixes upstream, unlike with Windows.
Ultimately if it becomes popular enough, Proton could execute the embrace, extend, extinguish strategy on Windows itself. We'll know it succeeded if one day Microsoft gives up NT kernel development in favor of shipping Linux/Proton, just the way they ultimately gave up on Trident and started shipping Blink/Chromium. Today Windows is a relatively small and shrinking percentage of Microsoft's revenue so maybe it's not out of the question in a decade or two.
Ultimately if it becomes popular enough, Proton could execute the embrace, extend, extinguish strategy on Windows itself. We'll know it succeeded if one day Microsoft gives up NT kernel development in favor of shipping Linux/Proton, just the way they ultimately gave up on Trident and started shipping Blink/Chromium. Today Windows is a relatively small and shrinking percentage of Microsoft's revenue so maybe it's not out of the question in a decade or two.