Eric S. Raymond carved Plan 9’s headstone 20 years ago:

The long view of history may tell a different story, but in 2003 it looks like Plan 9 failed simply because it fell short of being a compelling enough improvement on Unix to displace its ancestor. Compared to Plan 9, Unix creaks and clanks and has obvious rust spots, but it gets the job done well enough to hold its position. There is a lesson here for ambitious system architects: the most dangerous enemy of a better solution is an existing codebase that is just good enough.

Raymond predicted subsumation as legacy:

It may well be that over time, much more of Plan 9 will work its way into Unix as various portions of Unix’s architecture slide into senescence. This is one possible line of development for Unix’s future.

I wouldn’t call these a ringing endorsement of envy.