I think it could get big if people start making Nix distributions. I've been meaning to make something like NixOS for Grandmas that hides the config and tools behind a shiny installer and settings page and uses Nix as a control plane.
The best part is, there wouldn't be fragmentation of package maintenance between distros.
I'm not sure it will ever become mainstream. It might remain small forever, e.g. like the haskell/fp community.