Rust-coreutils may eventually become a suitable replacement for coreutils but it hasn't yet existed for long enough to iron out all the bugs and there's no real advantage to using it right now.
The idea is that Rust will make everything easier to maintain and improve in the long run, or something like that. It's somewhat plausible, but it's not totally obvious whether that will prove correct. So it's easy to suspect that there must be some other unstated motivations behind it, although to me there don't appear to be any in view.