Message passing to shared stateful objects/actors/processes is still a form of shared state. You cannot "replace shared state with message passing".
My understanding is that this is a profound misunderstanding of the actor model.
My understanding is that the actor model involves zero shared state. (That is, zero shared state between actors. An actor may or may not also not share state temporarily within itself (where itself is its current behavior), or temporally, between incarnations of itself (cf "Sussman and Steele ... concluded Actors were essentially closures that never return but instead invoke a continuation" from The History of Scheme" paper).)
The closest I currently see to anything that could be thought of as "state", but is not if you think about it, is an "address" that is rationally presumed to be the "mailbox" of some actor, or a "message" that is rationally presumed to encode some information. In reality neither is "state" bu