I think this is plugin territory. In my forum I have several threads where only admins can post updates for users. Not really a discussion topic, more of a change log or mini blog. If posts were to start merging the updates would get tangled.
Just one use case, but I'd prefer the core code not to automatically join double posts....or at least make it an option.
@julian even for the last edit, It will be a great addition, if you can specify message ("what was changed or why edit was made, etc.").
Edit history is a bit overkill for general forum 😉
