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" l

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.