My gut tells me that any benefits of adding Rust is massively negated by the addition of a second language.
If one wants to write Rust, there is always Redox and probably a bunch of other kernels.
I like Rust, but it's for sure an over hyped language. In a year or two, people will push for Zig, Mojo or some new pure and polished functional low level language. Maybe a Scheme or a Lisp? That seems to be what the cool kids use nowadays.
Or maybe we'll just replace the kernel with an AI that generates machine code according with what should be your intention.