Rewriting perfectly good code was a colossal mistake.

Not necessarily, but was the reasoning sound and have the tradeoffs been made? The website (https://uutils.github.io/) shows some reasonable "why"s (although I disagree with making "Rust is more appealing" a compelling reason, but that's just me (disclaimer: I don't like C and don't know Rust so take this comment as you will)), but I think what's missing is how they will ensure both compatibility and security / edge case handling, which requires deep knowledge and experience in the original code and "tribal knowledge" of deep *nix internals.

I do wonder whether people got down the article enough to see the list of bugs patched in GNU coreutils.

That "perfectly good code" that it sounds like no one should question included "split --line-bytes has a user controlled heap buffer overflow".