What Erich says is correct, please check out my blog post on the topic here: Ubuntu Core as an immutable Linux Desktop base | Ubuntu and the talk I gave with Ken on the topic at last year’s Ubuntu Summit. https://www.youtube.com/watch?v=ahWrhnjjYDk
You’re right that having a secure, immutable system has significant benefits but also restricts the freedom of users to truly customise or configure their distro. Our solution is designed to bridge that gap to some extent by offering a richer ‘composability’ story so users can replace subsets of the distro with their preferred alternatives, such as desktop sessions, or run them at different update cadences without breaking immutability.
That said there is no plan to replace the ‘classic’ desktop experience, simply to offer this as an alternative in environments where it makes sense (eg for those who focus on cloud-native development, newer users who want that stability, or thin-client and single purpose devices that want a smaller footprint). Indeed the components that make up Ubuntu Core Desktop rely on the existence of a classic system to be built 
It will be a while before we’ve fully refined the underlying architecture of Ubuntu Core Desktop to a point where a development preview image is made available but I’m excited to hear from the community about how it might fulfill their use-cases when we get there!