I think replacement for core utilities are not a problem, but as z3bra, I prefer tools that extends this set and I tend to rely a lot on the standard ones.

One example of extension I have built (probably the only one worth speaking about) is a tool to merge lines that share a set of identical columns. By merging I mean putting them on the same line, without removing the columns.
An "old" version of it is here: https://www.deadbeef.fr/projects/tools/f....1.md.html (this is the markdown files that I use to generate the man page).

Other than that, I would not be against a complete tool set that does what core utilities do (but differently), but with a more consistent API, and why not, expose a way to actually allow a shell to extract auto-completion through an option instead of relying on external files.
This basically imply that I don't care about stand-alone replacements for the core utilities, since by definition, they won't share consistent API, and thus "force" the user to learn every single tool. And I don't have time to waste with non-reusable knowledge.

I know, plan9 had that, and I should try it, someday...