You have to bare in mind the era Plan 9 was released in. The concepts of decoupling the OS from your choice of user space tools is actually quite a modern one.

You have to bare in mind the era Plan 9 was released in. The concepts of decoupling the OS from your choice of user space tools is actually quite a modern one. Back in the 89s and 90s things were a lot more fragmented. You had a whole plethora of home computers, from 8bit micro computers, whose implementation of BASIC were largely incompatible with each other, through to entirely different designs of personal computers (back when PC didn’t just mean “IBM compatible”). So machines like Atari ST/Falcon, Amiga’s various machines, Apple Macintosh/Lisa/etc, Acorn Electron et al. Even on the IBM PC, there wasn’t really the breadth of choice that there is now. So people did choose their OS based on what software why wanted to run. Even in the enterprise space things were more fragmented with less compatibility across the various UNIXes and Linux wasn’t really a viable option until the late 90s. It’s very easy to judge Plan 9 through the perspective of modern ideals and the expectation of POSIX everywhere but that wasn’t the case when Plan 9 was written. For its era, Plan 9 is has a number of very pioneering concepts.