You mistake my reasons. Popularity is not an end in itself. Popularity, in this case, is a requirement to capture meaningful workloads to test what is purportedly a research operating system.

The reason that Plan 9 failed as a platform for exploring OS design (not strictly speaking "OS research" in a narrow sense) is that modern workloads didn't run there.

Honestly, who cares which platform supports a handful of processes running the moral equivalent of xclock, a few shells, a 1980s looking terminal and the occasional build? This isn't a proxy for anything that anyone cares about, and all modern hardware is ludicrously overpowered to do that. Give me a modern machine, and I could probably write you something that simulated a decent-looking 1989-level experience in pure Python (I am not saying this is a good idea).

It rapidly became impossible to test any meaningful ideas on Plan 9 because what consensus reality regards as "software" doesn't really run there. This filters down into potentially bad decisions about design of OS and runtimes. You can't test out ideas that come from anywhere outside your narrow circle of "the Unix room and miscellaneous fanboys and trainspotters".

For example, is "everything is a file" a good abstraction for a modern graphical interface of any kind? Who knows, we'll never see one ported to Plan 9, just a bunch of bitblt-level fingerpainting.

If Plan 9 had stayed popular (there's that word) enough to attract enough users to matter, maybe we could have explored these ideas. Not necessarily building a dumb clone of Windows and doing everything the same way, but having enough interoperability to make it a plausible daily driver.