I use Cargo regularly and rustup it does make it easy to install. That's not the point. I'm not looking for a package manger. I want to be able to write scripts that make use of tools I can pretty much guarantee are already installed.
My primary point is that if I stick to coreutils, I don't have to worry about any of this. I can reasonably expect anything in coreutils to already be installed and available. Your functions seem like they'll work just fine, but either requires build tools to be installed, or the package to support binstall. For me, thinking about all these things and maintaining something like that isn't worth it when I can just use coreutils.
I totally understand. My .vimrc has shrunk from over a hundred lines to 4 lines in the last years. But I really think you are doing yourself a disservice. It is easier than ever (homebrew, nix, cargo, pip) to get nice software everywhere.