Starting with a stripped-out Linux kernel and a standard toolchain (whether gcc or clang) would start you light-years ahead of Plan 9 and still permit you to do the kind of research a lot of us (including former 9fans) would love to see. This really depends on an expansive view of "operating systems" (i.e. kernel + a lot of other stuff), but most of what you're saying is not exclusively the domain of the most privileged code.

As a bonus, you'd get a vast array of device drivers and be able to run actual computer programs used by more than a handful of people, which I hear is neat for attempting to build an innovative research operating system with some relevance.

I'm not going to argue that a standard Linux distribution is not a giant mountain of cruft, but imo it's the user space and system software that's alarming, not the kernel per se. And to the extent that the kernel is crufty: well, it's supporting orders of magnitude more things that Plan 9 ever did, and that's a good thing.

I share your irritation at all the things you outline, but you don't need to be running a wacky Thompson C codebase on top of an old kernel to fix any of these things.

I'm also far from convinced that there's any aspect of the Plan 9 design that should be accepted uncritically for a clean sheet new OS design. Saying "nothing has moved forward" is hyperbole, and plenty of things have changed since 1990, even if we accept your claim about OS stagnation as a given. I'm far from convinced that the insights of the Plan 9 guys circa 1989 are so wonderful and timeless that they are better than what could be done with a clean sheet design on top of modern hardware and a minimalist Linux kernel now.