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.