If future versions of Proton break compatibility with older Windows apps, you can use different old versions of Proton for individual games. Steam makes this very easy on Linux, but rarely is it necessary.
I don't foresee many Linux distros breaking compatibility with Wine, which is good, as some devs argue Win32 is the only stable ABI on Linux. [1]
I don't foresee legal issues either, as Wine has been around for 31 years, and its corporate sponsors have included Google in the past. I've seen no indication that the project is on shaky legal grounds.
Microsoft could always create a new API that Wine doesn't yet support, but good luck getting developers to use it -- they've tried many times, but not much has stuck, and most devs just stick with Win32. [2]