I should probably add that I haven't really daily driven an immutable OS yet, so this isn't from extensive experience or anything.
I'm big on trying to keep my system separate from userspace and also making my environment as reproducible as possible. I enjoy using Ansible and Nix-based environments to get some degree of that in more traditional distros, an immutable system would give me an extra layer of predictability I appreciate. I'm also a developer, so I enjoy having containerized development environments, which lends itself well to a stable base I don't have to worry about. Plus, like you mentioned, updates being more solid and reliable, often reversible, is always appreciated.
That all said, I totally get that for your average user none of that may mean much and that's fair game.