The main advantage of these distros is the declarative approach which allows for setup to be done in the same way as you would configure a Vagrantfile or Dockerfile.
In the way that Docker replaced/is replacing VMs for reproducibility, is there a chance these distros will eventually replace typical distros that are used in a DevOps setting due to their atomic and functional nature?
Some articles, including this one give some arguments why docker is popular:
In a nutshell, here's what Docker can do for you: It can get more applications running on the same hardware than other technologies; it makes it easy for developers to quickly create ready-to-run containered applications; and it makes managing and deploying applications much easier. Put it all together and I can see why Docker is riding the hype cycle as fast as I can recall ever seeing an enterprise technology go.
Moreover, for once the reality is living up to the hype. Frankly, I can't think of a single company of any size that's not at least looking into moving their server applications to containers in general and Docker in specific.
My personal view is that a tool could be successful if it will solve a (major) problem. For example, a half year ago I gave a presentation of docker in the company, the developers had some doubts until they experienced a issue that it was not possible to run all microservices on their local machine.
The same is applicable to the question:
Are NixOS and GuixSD the incoming Linux distros of DevOps?
It depends whether these distributions will solve a (huge) problem.
Rollbacks at nixos.org/nixos/about.html.