Not to be too harsh, but if that’s your (fundamentalist) attitude to software, remind me to argue strenuously to never have you hired where I work. Fact is you can’t rewrite everything all the time, especially the bits that power the core of a business, and has for a decade or more. See banking and pension systems, for instance.

I think you're completely missing the point. The problem being solved is not that coreutils is bad and thus they should be rewritten, the problem is that coreutils is not specified well enough to make new implementations straight-forward. Thus a new implementation written from scratch is tremendously valuable for discovering bugs and poorly documented / unspecified behavior.

For a business it's often fine to stop at a local maximum, they can keep using old versions of coreutils however long they want, and they can still make lots of money there! However we are not talking about a business but a fundamental open source building block that will be around for a very long time. In this setting continuous long term improvement is much more valuable than short term stability. Obviously you don't want to knowingly break stability either, and in this regard I do think Ubuntu's timeline for actually replacing the default coreutils implementation is too ambitious, but that's beside the point—the rewrite itself is valuable regardless of what Ubuntu is doing!