I'm surprised nobody mentioned this yet- a rust rewrite means you are more likely to invite contributions from a larger community.
I'd be really interested to know why you think that's the case. Somewhere around 3% of programmers know Rust, while over 20% know C. https://insights.stackoverflow.com/survey/2019#technology
Naively, I would have thought that a C project would invite contributions from a much larger community. Sure (as I've personally experienced!) the Rust community is friendly and helpful, but we're still small, so I'm curious why you think a rewrite would get more contributions.
Also for pure rust projects the development environment is easier to set up: cargo+rustc do everything vs. autotools+make+installing system wide libraries+ccache+gcc (just what I've seen in the wild).
Well, ok, if we're going by GitHub repos, C has 117k repos with at least one star and Rust has 11k. That doesn't seem to be that different than then number of programmers as a whole.
(FWIW, there are 104k starred C# repos and 2k F# repos, which doesn't seem all that out-of-line with the popularity of those languages as a whole.)