While I do agree generators offer some amazing features, I think they also offer some complexity that we are currently not ready to handle.

from a given for loop:

  for (x in function())

Please tell me when this for loop will end. You cannot.

If you instead did...

  next = fib()
  while (true) {
    number = next();
  }

You are instantly presented for every case in which this loop will terminate.

Now don't get me wrong, I DO fully agree that a generator is the correct method for doing this, I'm just pointing out another construct to allow you to do this.