Plan9 is funny because it's what UNIX might look like if the people working on UNIX understood UNIX, i.e. everything is a file and simple primitives are composed into complex systems.

They had the benefit of hindsight and bigger hardware, but UNIX got too popular and now we're struggling to move past it. It would have been interesting to see what the fourth try would be like (though looking at Go I would probably not completely like it).

yea, as sibling said. p9 was not possible on pdp11. what was possible there was .. v7 and 2bsd. see https://github.com/felipenlunkes/run-ancient-unix

p9 was done when "current state of unix" was already fixed in form of aix, sysv and bsds, it suffered the same fate as say beos.

BeOS was marketed, there was an attempt. But it was a harder sell. Plan 9 on the other hand was was kept as a research project only and was restrictively licensed in the 90s when it was actively developed.

i mean they did understand unix, both in that they created it, and created plan9 to be closer to their original vision.

but unix got widely adopted and you go with what sells not blue-sky woulda-coulda

Yes and no. Defining network devices in a different namespace, for example, was an early break from what might be called the UNIX tradition.