IIUC, the nix fallback paths provided a way for nixos-rebuild to bootstrap a new Nix evaluation of Nix in case the new Nix expression for Nix did not evaluate under the current Nix.

While a degree of paranoia around upgrade robustness is of course warranted, I don't believe we have needed this in this decade, and indeed the new nixos-rebuild implementation does not implement this functionality.

However nixos/modules/installer/tools/nix-fallback-paths.nix remains.
Should it?

cc @edolstra @NixOS/nix-team