If a Rust rewrite of Coreutils is not backward compatible, it is not a replacement and the current GNU Coreutils would still be needed for backward compatibility.

There is a long history of newer, better tools supporting backward compatibility to be a replacement for their predecessors:

- bash and zsh support backward compatibility to be replacements for sh.

- vim has compatibility mode to be a replacement for vi.

If the Rust implementation is not backward compatible, it should not be called "Coreutils".