"No rust code in kernel/dma, please" is a totally reasonable request though, once again it comes down to who supports it? When 99% of the kernel is written in C it makes sense to keep it C.

If you start introducing new languages that most maintainers are not anywhere near as familiar with, you create an unmaintainable mess.

Linux isn't a hobby project anymore, its critical infrastructure. You can just introduce changes because a few people think its cool.