@pluralistic it is confusing to read this because we have done basically everything you propose in the AT Protocol, and it has been live in the network for a long time now. ensuring that service migration is easy and seamless is truely seamless is literally one of the reasons we did *not* use ActivityPub.
You can read how easy the process is here:
https://whtwnd.com/bnewbold.net/entries/Migrating%20PDS%20Account%20with%20%60goat%60
On, I would mention, and independently run blogging service built on atproto! which bsky can't exclude/control