Nix devs seem focused on improving the ergonomics of the new command line, but it’d probably take time to flesh out the details. In the meantime, I think specifying the package source is a reasonable tradeoff given the features it enables.

I also suspect that keeping the package specification the same between installation and uninstallation time is harder than it might look, though. By its nature, Nix identifies packages by the contents of their definition. “nixpkgs#ripgrep” is merely a pointer to the current ripgrep package definition in the Nixpkgs repository. Therefore, “nixpkgs#ripgrep” will likely not point to the same package at the time of uninstallation.