If you're re-writing emacs at all, it's because you believe that a Lisp interpreter is the best way to extend a text editor. Or, at the very least, you want to execute pre-existing elisp.

I can see moving some core functionality currently implemented in elisp into Rust (or whatever typed intermediate implementation language). But there is a cost to having stuff in the core - user can't step into C functions with the elisp debugger.