Au contraire ;) Innovations of Plan9 landed in mainstream systems: Unicode, Namespaces => Containers. If you will, Docker is the result of a mainstream implementation of a Plan9 feature.

I was recently thinking, if you want to create a popular system feature, you just need to copy something from Plan9. IMHO the only reason nobody uses Plan9 is its bad hardware and userland software support. Also I've given up hope this will ever change, however it's a nice innovation testbed somehow.