> Right. It's not a very widespread use case, to be honest. You'd find that most would be N actors for M threads (where N <= M

What makes you think that? Having a large number of actors per thread is by far the most important use case. The Actor model is commonly used in communication systems where there are hundreds of thousands of actors per machine (often one for every single user). In this context, Actors are typically extremely lightweight and not CPU-bound. Instead, they mostly focus on network I/O and are often idle, waiting for messages to arrive or be sent.