> What would we do about all of the processes that fork but don't ever exec?

I speculate it isn't common, or at least it isn't required for ordinary desktop apps. Windows survives without fork().

> And isn't posix_spawn implemented in terms of vfork, which a lot of people like even less?

That sounds like a glibc implementation detail, not something required by the standard. I'm not an expert however. Simply, again I come back to: Windows can spawn processes without using it as an excuse to overcommit.