As I have been using Silverblue for enough time, I would say that splitting between the base OS and the apps is an important thing but atomicity/immutability of the base system is not so much.

For example: I also use QubesOS and it gives quite immutable-like experience while the base distro is a regular non-atomic Fedora.

By using flatpaks (or snaps) or tools like distrobox on a regular distro you will get a similar experience.

The main think is to cut dependencies between apps and the os and to be able to update them independently.

And then, when you have the apps separated, there are just not many reasons against choosing an immutable distro for the base system because it gives you additional bonus things as safe updates and rollbacks. But you can use a non-immutable distro as well if you want a specific or a niche distro (for example Chimera Linux or Alpine).