> There's a component that seems to be missing here which is preemptive task scheduling.

For Erlang, yes. For implementing behaviours (the point of my post), I don't think so (I sketch a "single threaded" solution towards the end).

I think one worker behaviour per CPU/core per stage in the processing pipeline is better than throwing thousands of processes at the problem and let the scheduler deal with it. This is what I got from Martin Thompson's talks (I linked to one of them in the "see also" section).