Immutable was adopted for Android because Google and the Android vendors wanted to lock down the platform, and because they always distribute their OS images and updates as binary blobs.
It offers no benefits to an open ecosystem like Linux, that you can’t already accomplish with existing security measures.
It offers some benefits to distro maintainers who are only willing/able to focus on the core system and delegate the rest of the software to distro-agnostic packages. That’s definitely an interesting niche and I look forward to it. But please note that whether the core is immutable is completely irrelevant in this scenario.
Generally speaking, if you want to use distro-agnostic packages you can do that regardless of whether the system is immutable or not.
And since we’re on the topic, if we’re borrowing things from Android I would love to have the application sandboxing and permissions. I think they’d be a much bigger benefit – to all distros, immutable or not.