So the thing that is the primary value proposition of the system, yeah?

I think the package managers themselves, with unprivileged operation, immutable profiles, atomic upgrades and rollbacks, "guix pack", etc are the main value propositions.

The OS thing is mostly a side effect.

Not sure about guix but the unprivileged part isn’t true of nix as you do need root to install initially.

It is only required because /nix is hardcoded (but only to make using binary caches possible). Later on, writing to this store is done by a nix user, and install can be issued by a normal user.

Maybe I'm misunderstanding, but isn't part of the point that the packages themselves are declarative and you're not beholden to some maintainer to get stuff? Wouldn't that mean writing your own... whatever it is your write to get software installed?

To install stuff with Guix you simply type "guix install foo".

From an end-user perspective, the main difference from other package managers is that you don't need root privileges (and that each user have their own "collections" of packages).

Both. You get to use packages others have written the same way as you do on arch. And you can also decleratively specify your build environkent, or override some attributes like apply a custom patch in a package someone else wrote. But you are just as not required to learn nix, as you don’t need to know how to write a .deb package to use ubuntu