If Windows would not have the amazing backwards compatibility it has, then would Quake still run on Proton, if it would not run on Windows today? And if so, how

If Windows would not have the amazing backwards compatibility it has, then would Quake still run on Proton, if it would not run on Windows today? And if so, how much effort would have the Proton team had to invest to make that happen? My guess is, an impossibly high effort that would benefit Quake, but perhaps not other games of that era. Microsoft is the reason Quake works on Proton still today.