plan 9's insight is pretty simple. the idea of unix was that everything was file, real or fake, so you could interact with anything using standard programs through the unified file system. someone asked the question "but what if i want to make my own service [and expose that through the filesystem]?" and the answer was "uh..." the insight added by plan 9 was simply that by abstracting filesystem layer out of the kernel and instead have the kernel be a simple multiplexer for networked file servers you could, in the presence of filesystem namespaces, have full control over every file from user space. you can see half-assed attempts at it in linux with vfs, fuse, and later their bastardized 9p2000.l variant, but because of the legacy hack that is the suid bit linux can't have proper transparent filesystem namespaces and similarly it can't do all the really neat stuff plan 9 can with exporting devices over the network because 98% of them are still weird special cases. you're right though, unix was good enough so people stuck with it, despite the shitty limitations.
Denmark