Sure some distros have many derivatives, but it's not obvious to me that you can stop that (see Android for a different open-source ecosystem, and how fractured that is), nor that is always a bad thing, as it allows experimentation which can flow up to the parents.
I'm not sure though "almost identical packaging work" is accurate, maybe that applies to Debian and Fedora (and their derivatives), but Gentoo and Nix are examples of doing something quite different (not to mention the different embedded distros), and so the "duplicate work" is anything but that.
