That's already a thing? It's called Ubuntu Core and is aimed at servers for now.

They are working on a desktop edition which is what you are probably referring to - it's a very interesting idea imho, they basically containerize the whole system, kernel and DE included. Will see how popular or useful it'll be, but I can imagine it being used in embedded devices for example. In my understanding it's going to be like most immutable distros, but with snaps instead of flatpaks.