This is an interesting argument and perhaps has merit, but as a matter of fact: It became open source in 2002, 10 years after the initial 1992 release; albeit under a GPL-incompatible license. In 2014 it was released under the GPLv2; 22 years after the initial release. Either way; less than 30 years.
"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