Which is unfortunate, because Plan 9, while carrying some awesome "beyond-POSIX" ideas, was born dead.

I think both GNU/Linux and Windows are not dead, just mature. Unfortunately, their maturity means it's really hard to teach them new tricks (for example, like non-hierarchical filesystems, which were recently discussed on HN), so, when considering all those fancy new (and sometimes not so new) ideas, those mature OSes certainly feel somehow old and limited.