Well, the last major revision to the Single Unix Specification was released in 2008. Ever since then, Linux has basically become the defacto POSIX standard. So while glibc may not rule the roost, most POSIX OSes bend to the will of the GNU userland and the Linux monoculture in many ways. It's not really like the old days with Sun, IBM, DEC and even Microsoft with Xenix. POSIX is being washed away a little more every day.