I think Microsoft, and everyone else, could benefit a lot from being a POSIX compatible OS but I don’t think it would ever happen. They would end up maintaining a compatibility layer forever which would hurt them more than help.

I think they could make a new “Windows” that has nothing to do with the current OS, is POSIX compatible, and use that for whatever the future of personal computing is. However I don’t think they could use many of their current developers who are so deep in the Microsoft way of doing things they’d never innovate with something new.