I'm not just a fan of changing something unless it's broke. I have nothing against rust, per se.
But changing the core utils from very known, very old, highly battle tested and maintained code to rust utils, without proper testing (already there are many critical bugs reported), just isn't something I can agree with--particularly when we're talking about core utils.
Also the original core utils maintainers are skilled in C/C++. Now to say they need to learn rust to maintain the same toolset, is not exactly fair IMHO.