There seem to be some confusion about what POSIX is. It is a common agreement on the very minimal set of things any OS should implement anyway, and a common way to do it. That's it. It does not define a complete OS interface for the modern world. Any OS which would try to be "strict POSIX and nothing more" would be missing several important parts to be really useful.
The main distinction is how extensions and things going outside POSIX are handled. Either, like Linux or *BSD, you try to evolve things while keeping the same spirit and adding stuff that blends in with POSIX. Or, more like Haiku or macOS, you have the POSIX API on one side, and another completely independant API next to it (even implemented in a different language - C++ in our case, Objective-C in macOS).
So, POSIX is a good platform to build on. But it's just that, and you need to build something on it.