OS/2 was competing on the enterprise market, I don't really see it as a good comparison to the Linux/Proton situation, yet for some reason I see it brought up a lot in these discussions. Why would OS/2 come to mind and not the successes of https://en.wikipedia.org/wiki/Embrace,_extend,_and_extinguis... or https://en.wikipedia.org/wiki/Windows_Services_for_UNIX ?

Because those things were only to get DoD contracts, "Supports POSIX" checkbox.