Activitypub makes it next to impossible to "move" an instance to a new domain.
Every post/comment/and user is uniquely identified using the domain. In the eyes of ActivityPub changing the domain just makes each of those things a completely new thing.
You can set up a new service at your new domain and potentially get most all your users to migrate but they'll be leaving behind their entire histories and as a "new" fediverse user they'll only be discoverable via the historical posts for as long as the original server is reachable.
But, the theory goes, you're not supposed to be reliant on third parties as you should be in control of your own domain (or within a few degrees of the person who is).
Large instances are what are antithetical to decentralisation.
Of course, the reality of it is that, it just hasn't worked out like that.