Is it conceivable to replace the various GNU tools with Rust equivalents? Yes. Is Rust a good fit for the job? Yes. Is it a wise investment in engineering time? Absolutely not. Would this sort of Rewrite-it-in-Rust campaign get much traction? Probably not.
I think you would need a better reason to duplicate several developer-decades worth of work than to prove Rust can do it or attract developers to improve the standard GNU tools.
Besides the duplication of efford issue, if you want people to switch they'll need a drop-in replacement... meaning your Rust program needs to be bug-for-bug compatible with the C version. Anything less and people will lose confidence in the Rust program because they'll run into subtle bugs where they've relied on an undocumented aspect of the C program's implementation (relevant XKCD) and are surprised when Rust behaves differently.
If it isn't a drop-in replacement that means people will need to rewrite all their old code to work with the Rust equivalents, which is a bit optimistic. People also have inertia, and if they've become an expert in using GNU
grep they probably won't want to throw that knowledge away and to learn
ripgrep from scratch. It's just a lot of code to rewrite for no tangible benefit.
I love Rust as much as the next person and it has been my favourite language to use in day-to-day applications for about 5 years now, but I don't think rewriting the GNU tools would be a good way to spend your time unless it's purely a hobby.
That said, the
coreutils project seems to be taking on the challenge.