Hmm yes. That is a great point. I believe that I’ve also seen something similar brought up in that GitHub Issue for Mastodon a long time ago, but alas, I just got off work and I haven’t had a chance to update myself. Consistency is /quite/ a tough problem, if not one of the toughest to be able to viably solve.
On the other hand, I think that perhaps a bad implementation might be acceptable if done well. We would never be able to resolve/rewrite external links, nor be able to effectively rewrite history outside of an instance. I think it would be possible to migrate /some/ things, even if it’s not everything. Breaking some things (e.g. ignoring replies) makes even more sense when considering a host instance disappearing entirely.
There are tradeoffs of course, and right now I am severely ignorant anyway. I have no educated solutions to this barrier to entry.
Also I linked to your reply in a reply here :) :
https://social.nerd.net/@fooderick/111825715301093492