IMHO, the problem is more subtle: nothing on the internet will stay forever, if you find a piece of information you like to save forever, you should save it locally AND with something like internet archive. A community can transfer to the same community in another server with proper forewarning. Finally, mastodon introduced the ability to move your account to another istance manteining your followers for quite a while now, maybe lemmy can find a way to introduce something like that too