I don't know if mobiledoc pioneered this approach of separating the data model but it was the first time I saw it and it felt right.
Glad to see so many editors do this now.
In CKEditor 5 we took perhaps the longest possible road so a tree structure with Operational Transformation support and couple of other goodies. It costed us a 3 years of work, but it was worth it. We proved the concept and validated the results by implementing the Letters app (https://ckeditor.com/letters/) which supports real-time colaborative editing for real (with selective undo, commenting, displaying selections of other users, etc.). We'll now be working on adding more features like e.g. tables support which rise the bar for the data model and the whole architecture even higher, but we're very optimistic seeing the results to far.