You mean like https://wiki.nixos.org/wiki/NixOS_Containers ?

sounds like you want qubes os https://www.qubes-os.org/

depends whether you consider rootless Docker "cheap". I tried running ZeroClaw in a Nix-derived Docker (spoiler - it was a bad idea to use ZeroClaw at all since the harness is very buggy) and there is still a potential for container escape zero-days, but that's the best I've found. also, Nix's own containerization is not as hermetic as Docker; they warn about that in docs

If containers are safe enough for ur use case then just use nixos containers they just a few more lines to setup in a regular nixos config

If it isn't enough there's microvm.nix which is pretty much the same in difficulty /complexity, but runs inside a very slim and lightweight VM with stronger isolation than a container

That's hard given most apps have dependencies and often share them.

It will always look like curl is available or bash or something

What's wrong with another user account for such isolation?

They can be isolated to namespaces and cgroups. Docker and Nix are just wrappers around a lot of OS functionality with their own semantics attempting to describe how their abstraction works.

Every OS already ships with tools for control users access to memory, disk, cpu and network.

Nix is just another chef, ansible, cfengine, apt, pacman

Building ones own distro isn't hard anymore. If you want ultimate control have a bot read and build the LFS documentation to your needs.

Nothing more powerful than the raw git log and source. Nix and everything else are layers of indirection we don't need