It was never meant to be. Just like UNIX initially, it was a research operating system. UNIX turned out to be an accidental runaway success, and many of Plan9's greatest ideas (like synthetic filesystems, bind/union mounts, its threading library, the 9p protocol, etc) found their way to more "practical" systems.
It also did have a commercial counterpart in Inferno (mostly finding use in products like telephone switches). But nowadays both remain mostly as toys for enthusiasts. You can get work done, if you're patient enough, and have the freedom to ignore things like Zoom calls or Excel files.
Plan 9 was also available commercially for some time.
My take is that plan 9 is basically a "pure" rewrite of a unix/posix OS, that takes many underlying principles of unix to their logical conclusions, without the taint of the incremental evolution of the existing systems, yet nevertheless informed by the lessons learned operating real OS systems. The incompatibility is why it hasn't moved forward, though many of the good ideas that are compatible with systems like bsd/linux have since been ported over.
It is really useful exercise to consider, in hindsight, how could we have built a better unix? And a bunch of really smart people participated and came up with some great ideas.
I'm interested in hearing what it's like to use it more seriously; my biggest doubts are around collaborating with people on conventional OSes.
I think I'm convincing myself in this thread that it's not for me - but of course I think it's great that others are using it, I don't want to sound too negative.
If you’re interested in it, set it up on a Raspberry Pi, connect to it with drawterm from your daily driver. Experiment with adding a second machine, netbooting it as a CPU server etc. That way you’ll look at Plan 9 from the point of its strengths, not weaknesses.