Also a Nix/NixOS user/contributer. I feel like Nix+nixpkgs could become the universal packaging solution adapted to build deb, rpm, etc packages.
Through nix-env it even already has the idea (albeit discouraged) of imperative package management like apt.
Having spent a while with NixOS, running containers for user applications seems like using a cannon to kill a fly. The 'simple' alternative is to drop FHS altogether - which containerization is kind of doing in a roundabout way by isolating the app filesystem from system fhs.
As for it being for developers only... I get that perspective. Debian/Ubuntu packaging is also hard, AUR packaging has its quirks. A lot of this is hidden behind the package manager, wheras with NixOS it is more obvious.
The killer idea for NixOS would be to make package management for Joe Q Public as easy to use as apt while. Tools like MyNixOS[1] are emerging which might bridge that gap.
The one advantage Flatpak provides for me over Nix is containerisation. Not the bullet-proof kind, which allows you to run malware, but of the "reasonable" kind, which stops apps from storing to any directory they like just like that (only chroot level of "security"/isolation would be fine for me).
When there's a package manager / runtime that does both then I'm extremely interested.
It looks... somewhat abandoned, but I'd wager it still works today. Failing that, setting up a shell alias to launch a regular binary in bubblewrap isn't too hard either.
It's a long standing goal of mine to put together a distro with the system management aspect of NixOS, the isolation of bubblewrap and the lightness of Alpine. I'm gonna start this project when I have time™.
