You can imagine the physical world version of "message passing" already because you already live in this world. Messages come to your house or apartment building or whatever
For Bluesky's "shared heap" architecture, you'd have to build a whole addition to your house for everyone's mail
That's exactly why running a Relay or AppView is expensive: you're building an addition to your house for all the world's mail.
Eeep! That ain't cheap. That's why I'm saying: decentralization also means the ability to *scale down*.