Message delivery is presently centralized. As more nodes are added, message delivery grows quadratically. Inherently, ATProto's approach relies on only having a few meganodes delivering.
This means that "credible exit" is still a viable path, but "full decentralization" is not possible unless Bluesky pivots to incorporating "message passing" into its architecture in some way or another or accepting a great degree of lossy replies, which explicitly is stated as a goal to avoid.