Windows as a platform might become irrelevant, but the convenience and robustness of the Win32+D3D API combo for game development simply has no equal on Linux (except Proton of course). I trust Valve and the Proton team to have a fallback plan though. The desktop Linux community (if it can be called that) is too fragmented to come up with a solution. In this case more centralization (of development efforts) would be a good thing. But I don't see native Linux ports taking off for at least the next decade. Proton is just too good for that to happen ;)