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.
I will never understand why that 2 lines in the terminal bother anyone. It's a free service. Free as in free beer. If you use the terminal, you are tech-savvy enough to disable it, if you don't use the terminal, you'll not see it anywhere, at all.
While we are at it, they advertise all of their products like that in the terminal, not just Ubuntu pro. Landscape and microk8s both appear like that on an ssh login.