Well so if servers were able to identify themselves to each other using a public key, they could change domains whenever they wanted and just update the others

Well so if servers were able to identify themselves to each other using a public key, they could change domains whenever they wanted and just update the others with a signed message. You'd still use DNS to navigate, for user identifiers, but a change wouldn't break federation. Unfortunately this is not in the spec and it is a bit involved to do.