"To get people to switch to X competitor, it needs to not be better, but 10 times better"
I am sure I hacked that quote up badly but, the point is, sure plan9 is cool, but it wasn't better-er enough to get people to switch.
Instead people brought a lot of plan9 concepts TO other systems. That's a good thing.
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.
— Eric S. Raymond