> Plan 9 failed, in a sense, because Unix was simply too big and too entrenched by the time Plan 9 came around.
The Unix renaissance in the ‘90s had a lot to do with open source source OSes such as (primarily) Linux and the BSDs. Until then we had a bunch of corporate unixes, no two alike but expensive and kernel hacking was basically off limits. With the advent of i386 we had affordable & powerful computers that could run “real” OSes and us hackers were hungry for an open source OS. Computing history might’ve been different if plan9 was made open source before Linux became available. But that was not to be.
Note that Linux is basically getting the whole Plan9 feature set added to it, if in incremental and unplanned ways. AIUI, a kernel feature for implementing block devices in user space was added quite recently.
IMHO the kitchensink like approach of adding more features to Linux misses the point of the simplicity of plan9. Representing & accessing resources as file systems was the real insight. If the OS provides N different ways of doing something, it has to continue supporting that which has a real cost. I once compared compiling plan9 & Linux from scratch on the original RaspberryPi. 1 minute versus many hours. Granted that the Linux kernel does a lot more and supported a few more devices on 'pi and plan9 pushes more drivers in the user space but even compiling everything on it took about 4 minutes. That included Ghostscript and two editors and the windowing system and more.
Linux has to support its kernel syscall interface, so that Linux-native programs continue to run. Pretty much everything else is up for grabs - but if you want native Linux software to interoperate within a more Plan9-like system, adding features that allow for implementing/managing these kernel-provided interfaces in userspace is helpful.
The Unix renaissance in the ‘90s had a lot to do with open source source OSes such as (primarily) Linux and the BSDs. Until then we had a bunch of corporate unixes, no two alike but expensive and kernel hacking was basically off limits. With the advent of i386 we had affordable & powerful computers that could run “real” OSes and us hackers were hungry for an open source OS. Computing history might’ve been different if plan9 was made open source before Linux became available. But that was not to be.