You're 100% right, I think we do a lot more to stop content rotting like this. Now it's half gone, with any contributions from other instances unable to federate, fracturing any more conversation.
Maybe we need to do two things:
- ensure that popular instances aren't single-admin shows (so there needs to be a team with the skills, finances and motivation to run if one or more disappear, or at least the credentials in escrow so someone can step in)
- have some system to backup/migrate content
The migration one is tricky, because ActivityPub is very tied to urls. I guess that you could make dummy account for the conversation you brought over, so you'd at least have a read-only record.