Guix already has as many packages as Nixpkgs did back when I started using it, and like Nixpkgs, the package set is growing exponentially. What you say is true but it also seems to me like for many, Guix already provides a very useful starting point.
I think it’s “only” that yeah. Both have fundamentally the same capabilities. Nixpgs may provide more out of the box library functionality than guix, I’m not sure, but as far as I know there’s nothing possible in one that isn’t possible in the other. Would be glad to be corrected by anyone with more experience, though. I’ve run NixOS for years now, but have only dabbled with guix.