NixOS/Guix still give users the control to change anything at any time. That's because they aren't image based and instead they achieve immutability with the Nix package manager (symlinks and a declarative system configuration file).

I agree with immutable OS on servers being great, but I also believe immutable systems are a good choice for desktops. Especially managed desktops (eg. my moms) work well with an image-based OS. Flatpak is often enough for those few gui apps and there's less risk of automatic updates failing (eg. pulling power plug while updating).