We do not need “HM”. We need user-level configuration management - with decent quality, contributed by people who actually read some of the systemd docs before writing systemd services.
To that end, we might benefit from making our module architecture more “system-agnostic” - i.e. one layer for generating configuration, and another layer for deciding where that configuration goes (in /etc, in /home/whomever/.config, …) The first agnostic layer can be consumed both within and outside nixpkgs, enabling the simplification of projects like hm and nix-darwin, or even used outside of nixos in the context of system-manager.
I believe [RFC 0163] Portable Service Layer by svanderburg · Pull Request #163 · NixOS/rfcs · GitHub proposed something similar but slightly different.