At a macro level, there are plenty of syscalls not wrapped by glibc that are used regularly in userspace. Then there's the micro level assumptions of behaviour.

At a macro level, there are plenty of syscalls not wrapped by glibc that are used regularly in userspace. Then there's the micro level assumptions of behaviour. Look at the list of "doesn't work on WSL" software and all the bug fixes to get boring, every day stuff to run on it (or LX zones, or BSD personalities, or…)