I don’t think there’s any further down to go, but there’s probably room to go up to distributed computing. IIRC, some early versions of Go when it was in alpha had channels work across machines.
I think there is. How do you describe data parallelism? coroutine are too big when all your function call are doing the same things, but on a different data (think ISPC, shaders, cuda). There is still one more step in parallelism where coroutine cost to much :-)