Microsoft can't do shit against WINE/Proton legally, as long as either project steers clear of misappropriated source code and some forms of reverse engineering (Europe's regulations are much more relaxed than in the US).
The problem at the core is that Linux (or to be more accurately, the ecosystem around it) lacks a stable set of APIs, or even commonly agreed-upon standards in the first place, as every distribution has "their" way of doing things and only the kernel has an explicit "we don't break userspace" commitment. I distinctly remember a glibc upgrade that went wrong about a decade and a half ago where I had to spend a whole night getting my server even back to usable (thank God I had eventually managed to coerce the system into downloading a statically compiled busybox...).