Actors in Erlang seem more emergent to me than fundamental. To first order, Go and Erlang use similar concurrency strategies with cheap green threads and channels for communication. The main difference being that Go has channels as separates objects that can be passed around (leaving the goroutine anonymous), while Erlang fuses the channels into the process. In this respect, they both have similar levels of "actor-ness" in my mind. The biggest upside that I can see to channels being separate is that a goroutine can have multiple channels, they can be sent around (though so can PIDs), etc. This matters a lot because the channels in Go are statically typed, while Erlang/Elixir's dynamic typing makes this less meaningful since you can send anything on the same channel.
Of course, I supposed if one defines an actor as a sequential threaded process with a built in (dynamically-typed) channel, then Erlang is actor-based, so maybe I contradicted myself. In Erlang/OTP, the actor part is important to the supervision tree error handling strategy, which I think is the biggest upside, but it's not obvious to me that you need the channel and the process totally fused together to handle that.
Specifically in response to your "cheap concurrency good, actors bad" thought, my hypothesis is that actors are at least the more natural strategy in a dynamically typed language, which is why they seem to work well in Erlange/Elixir, but don't see much use in languages like Haskell (or Scala, it seems, where I at least though the Akka actors were kind of awful). Meanwhile, channels seem to fit better with static typing, though I can't quite put my finger on why.