This looks like a lot of work for what amounts to very little end-user benefit. Honestly, I'd rather see more progress on the Emacs Lisp/Guile migration than on a putative C/Rust migration. At least with the Guile switch, it's something that will have an obvious (and probably positive) impact on the people that use the editor.
It seems a bit rude to complain about volunteers for doing something they want to do rather than what you want them to do. I imagine that this, for the most part, isn't even opportunity cost: the people working on it probably wouldn't be contributing to full Emacs (or the Guile effort) anyway. And this sort of effort seems like it could be an on-boarding experience to improving the original code base, e.g. in the pro…
In any event, I think we would do volunteers a disservice if we placed them in a special class to which we could only provide positive feedback. (Maliciousness is never appropriate, of course.)