Yep, that was my whole point. I'm curious about how much (if any) an improvement there would be by switching an existing OS (namely Linux) to a "more modern" language like D or Rust, which are purported to be better for writing OS kernels. You're not going to find that out definitively by writing a research OS in them; you need to write an actual OS kernel in it, which has a counterpart already written in C, so that you can compare directly.
Writing a research OS in D/Rust seems to me like someone claiming (before Tesla/LEAF were released) "electric motors are better for cars! Electric cars are superior to gas-powered cars!", but then instead of making an electric car that's actually comparable to a gas-powered car, they build an electric tricycle (velomobile) out of mostly bicycle components that weighs 40 pounds and seats one person and has a custom carbon-fiber fairing and a max speed of 30mph. Yeah, it's nice that you can build that, but it doesn't prove the assertion at all, because the vehicles are totally dissimilar and incomparable.
Do you consider iOS, Windows, ChromeOS, Android, IBM i, z/OS, Unisys MCP toy OSes given their architecture and minimal use of POSIX?
POSIX only defines the necessary to write CLI and daemon software, nothing more.
What I consider real software is a bit more than that.
I never felt the need for POSIX when I am not using C.