I'll say that if you could run a web browser on Plan 9 and maybe there was a POSIX or a Linux compat layer, it could've become popular. It'd be a boost to porting efforts if it was a usable desktop OS. Give me the best OS, but if I have to write a whole desktop/server stack on it to make use of it, I'll stick with what I have at hand.

A plethora of software is available for Unix and it's why it's here still. Had I the chance to switch to, say HaikuOS, without a big/complete productivity loss, I would.