I'm curious what language you have used coroutines in where it works out that way.

In my experience of using fibers in ruby (which I _think_ are substantially/exactly the same as coroutines?), it hasn't been like you describe at all -- promises, or something other higher level abstraction, are still pretty neccesary to do useful and comprehensible things with them. So my experience colors my understanding and I think "What's he talking about, that doesn't make any sense."

More likely, you have experience in a language where things are done differently enough that it all comes out different.

What language?