How so? GP's post may be a little bit too harsh, but it's definitely well written and coherent.
Even dead things are worth studying, if for no other reason than to explore the question of why everyone isn't using them (in Plan 9's case, it has a lot more to do with accident of history than any technical limitations... Once POSIX standardized what a UNIX-like OS that managed processes and allowed IPC looked like, everything close-to-but-not-that became also-ran, no matter its technical merits. Esperanto is a fine language, but useless if almost everyone you meet knows English instead).
Strange that.
I will agree it's not for everyone, but I still wouldn't put my mom on Linux either (not for a desktop... absolutely not).
Also intellectually incurious: assuming that P9 expired due to POSIX eating its lunch. My personal take on it is that P9 expired partly due to bad licensing decisions but mainly due to a preposterously shitty attitude to Other People's Code. I vividly remember people in the Unix room clustered around the one Windows machine there that could do radical stuff like "run games" and "run a browser". If just a bit more effort had been put into Howard Trickey's APE Plan 9 might well have kept evolving and become useful, but NIH was more important. Or "Invented Here, But Not By The Right People" (C++).
POSIX could have been an asset to Plan 9 - a target to track instead of having to emulate a bunch of disparate Unixen.