> How would that work in reality? Re-use the existing tests to build a new kernel from scratch? Sounds like a very far-out idea that wouldn't help with any of the current problems, but I'm happy to entertain the idea and hear your reasoning here.

While I would tend to agree that a full production replacement would be such a massive undertaking as to be impractical, https://github.com/nuta/kerla does something very like that - Linux userspace ABI on an all-new Rust kernel. (And even at this small scale, I find it mind-blowing that this worked)