IMO the only/main reason to rewrite C-parts in Rust is to refactor and untagle the code to generate better incentive for people to work on it, because from what I heard - some C parts of Emacs are of nightmarish complexity, and there are literally only a few on this planet who even want to deal with it.

And the same time, I don't think there's enough people who can be persuaded to think that in a decade or two those Rust-rewritten parts wouldn't become problematic either - who can promise today that Zig for example isn't a better choice for that? Or maybe even some close-to-metal Lisp variant?