If P9 had been able to run a reasonable number of computer programs not written by people at the Labs, it might have survived or at least been more influential beyond "UTF-8, some odd remnant uses of 9P, and /dev/proc". The gratuitous incompatibility assured that the other interesting ideas were buried.
Plan 9 has had a very active development community during the past decade, nothing is preventing people from coming up with interesting ideas and patches, even to APE if that floats one's boat. Mixing different contrasting cultures usually just doesn't work, if you want POSIX, use a POSIX system, if you want Plan 9, go native, i.e. how the system was intended to be used.