I think relearning all the mistakes of the past over again would be a set-back. Educational for sure, but quite a set-back. Probably better to just be more vigilant about removing old unused code from the existing kernel which they already do to some extent. The code and git logs already have a lot of comments that explain some of the lessons learned. I have not seen a need to rewrite it from scratch. With time parts of the kernel will probably get rewritten in Rust where it makes sense.