Linux

Partly yes, but just installing a package without running into conflicts does not yet guarantee a working system. You have to cater for the right configurations too, for example when you think about a corporate setting with all kinds of networking whoes (like shares, vpns and such). I think you could get this to work with Nix somehow, but you want to test these things beforehand, and if you do so using images then you have the thing to ship to machines in your hands already, there's no need to compose the OS and configurations over and over again for every machine.

Another aspect with non-atomic OS composition on the target is that you have to deal with the transient phase from one state to the next. In this phase all kinds of things could happen, for example an update of nvidia drivers would render cuda disfunctional until the next reboot, as the userspace and kernelspace parts do not fit together anymore. With something like any of the fedora atomic variants, transient phases with basically undefined behaviour do not exist, and the time the system is not guaranteed to be in working order gets reduced to just the reboot.

Nix is cool and definetely better than any traditional package manager. But it is not an ultimate solution, to be honest so far it seems to me like it is living in a nieche of enthusiasts that are smart enough to put up with its unique declaration language. And below that niche you have ordinary linux users that may just be happy with silverblue without any modifications, and above that niche you have corporate doing their own images in CI/CD, CoreOS and all that jazz.