You're misunderstanding, and it's a great opportunity for me to clarify:
I want a graphical installer for applications within NixOS.
Currently NixOS applications are added by editing the configuration.nix, with more specific tooling avaliable via flakes and home-manager. We agree that this is cool and good.
What I want for consumer use is NixOS, with an interal graphical installer that handles updating configurations in the background; ideally forming a no-terminal-necessary UX for consumer users.