Well home-manager is a downstream project, to really endorse it, we would need to make it an upstream project (i.e. merge it into nixpkgs probably), since at the moment nothing is preventing a nixpkgs commit from breaking home-manager (it probably does so occasionally).
And upstreaming home-manager is nontrivial, I think, since you’d need to, for example, figure out how to eliminate the code duplication between nixpkgs nixos modules and the frequent reimplementation of said modules in home-manager for user services.