That’s a good set of points and I agree. I am starting to think this technical difference reflects underlying social / experiential differences:

• ActivityPub “clumpy” federation (like a region of city-states) where your view of the network is based on who you and others in your instance interact with - interoperability, but not homogeneity of content or interaction

• Atproto “swarm” federation (like a pool of taxis sharing livery, with possibly-but-not-necessarily independent operators), endpoints are exchanging data to compose a single virtual platform out of replaceable interoperating parts - federated but not decentralized, having a primary network (the relay service) holding everyone’s experience together

To me the former feels like it encourages a spirit like original Internet communities (MUDs, BBSs, message boards) while the latter produces that of branded app platforms (Twitter, Facebook, Snapchat).

Mirroring the entire network is what makes it a friendly experience for newcomers, IMO. In my own Mastodon instance I have to subscribe to a big relay (infosec.social) so that a reasonable proportion of replies from other instances I don’t happen to be happening populates into the feed.

I suppose you could say AP makes this optional, but that seems like a reasonable design choice to diverge on rather than a critical flaw in my opinion.