I disagree that data hosting is the most important part. I think switching "servers" and still being part of the network is the most important part.
You are right that ATProto is open-source, but you ignore the fact that there's only one company controlling the development of ATProto. You can decide to close it down tomorrow, or you can decide which contributions to accept and decide the general direction of ATProto at your own discretion.
This is all fine, you can do whatever you want, but don't try to hide behind detailed technical discussions when comparing ActivityPub to ATProto. Because the difference is much more fundamental than the question how much RAM my server needs if a post goes viral.
Don't say Bluesky is decentralized and federated, because it is not today.
I understand that you are not one of the founders and you probably see this from a naive technical perspective. But working for a company that needs to show VC-level returns, I think you are being ignorant of how the future will look like.
Finally: If the team at Bluesky sincerely thought that ActivityPub is not good enough, why not work with the ActivityPub team to improve the standard and address the challenges? Or, why not give ownership of ATProto to an independent standards organization? I understand that you don't want to do this because you want to control how to develop the protocol, you want the speed and flexibility. That's fine. But don't act like ATProto is the same as ActivityPub in terms of openness.
Sorry for the long post :-)