The user can download their posts.
When a host goes away, where do you propose the posts previously hosted there now get hosted?
One option is for the poster's new host to have all the old posts uploaded ... how then to previous other posts that point at them, or reply to them, get updated?
I'd genuinely be interested to hear your suggestions for how these technical issues could be resolved.
When a reference to a post becomes invalid because it's underlying host is gone, the other host holding those references tries to find new duplicate posts with the correct signature, and if it finds them, updates the broken old references to point to these new duplicates. If it doesn't find any duplicates with a matching signature, it leaves the references broken and has some cooldown before it tries to fix them again.
When a server stops federating, the federated posts still exist in the DB of all the remote instances that recieved it. They're still indexed and searchable from there. You could move those posts into the DB of a new server, but they'll still "originate" from the old one. But you can carry on and interact with old threads.
There's no real foolproof mechanism for updating posts, although there is an AP Update activity, much like deletes there's nothing forcing an instance to respect it.
Not pretending to know anything about the system feel free to point out why this wouldn't work. I'm just approaching it from a user perspective that having to lose posts to move server is bad for users and having to choose to nuke all posts in the case of shutting down a server is bad for admins.
You have outlined two scenarios. One where the old posts are deleted, and in the old posts are moved to the new host.
In the former case, the "referencing posts" start pointing to 404. Maybe in the latter case, we can also just have the referencing posts also point to 404.
A partial solution is better than a full solution.