Oh, sorry for not addressing this question more specifically early; I hadn't realized it was such an open question. This part of the global actors proposal came from the observation[*] that the presence of @ObservedObject, or other property wrappers that had to be used from the main actor, were a strong indication that the enclosing type itself could only reasonably be used from the main actor, and the general desire to lessen the @MainActor annotation burden.
Doug
[*] Hah. Hah.