“Plan 9 failed, in a sense, because Unix was simply too big and too entrenched by the time Plan 9 came around.”
UTF-8, /proc, union filesystems and per-process namespaces all came out of Plan 9 or got perfected there. Networking is just files you can script from a shell. Linux spent twenty years bolting on worse copies under names like containers. The better design lost to the one already installed. It usually does.