Rust will allow to safely invite a broader range of contributors, because there are so many things you don't need to check. This also means a smaller number of required tests, and because Rust uses higher level constructs that C, more productivity in general.

So basically, on the long run, more people, able to do more things.

Besides, on of the goals of the main implementation is to stay simple, which is hard to do in C. For those reasons, and because of the potential for unreliability and security, CPython is quite slow.

We can't optimize it, because it would make it too complex.

But with a rust implementation, one can hope to suddenly be able to apply more optimizations.

It's all theorical of course, but it's a nice hope.