[matrss]

I think the storage optimization aspect is secondary, it is more about keeping control over your distribution. You need processes to replace all occurrences of xz with an uncompromised version when necessary. When all packages in the distribution link against one and the same that's easy.

Nix and guix sort of move this into the source layer. Within their respective distributions you would update the package definition of xz and all packages depending on it would be rebuild to use the new version.

Using shared dependencies is a mostly irrelevant detail that falls out of this in the end. Nix can dedupe at the filesystem layer too, e.g. to reduce duplication between different versions of the same packages.

You can of course ship all dependencies for all packages separately, but you have to have a solution for security updates.