The Unix team created Plan 9 to supercede it, so that's the logical choice. BeOS had a lot of fans, too.

(Completely unrelated, I just discovered that the Plan 9 logo bears a striking resemblance to the Go logo [see the Wikipedia link]. Given Rob Pike's influence, I'm sure that is not a coincidence.)

[1] https://en.wikipedia.org/wiki/Plan_9_from_Bell_Labs

I'm also wondering this. I have not seen an OS superior to *NIX systems, UNIX in particular, when you add the context of era.

Any of those designed at Xerox PARC, or security minded ones designed in the 60's like Burroughs.

So am I. Maybe BSD?