Why almost?

if you mean nix the state declaration, it is a ilusion. when you have packge for debian 12... you just install it. when you have debian 13, you need the package for debian 13 and there's no way around it. nix lies that you don't have to worry about all that. which is only as true as is true with packages. if you can replace -12 with -13 in a non nix setup, your nix "package x" will still work. otherwise you will have to deal with it, just with more layers.

if you mean nixos, that's just starting yet another distro maintenance issue from scratch