Design indeed becomes the bottleneck, I think that this points to a step that is implied but still worth naming explicitly -> design isn't just planning upfront. It is a loop where you see output, see if it is directionally right, and refine.

While the agents can generate, they can't exercise that judgement, they can't see nuances and they can't really walk their actions back in a "that's not quite what I meant" sense.

Exercising judgement is where design actually happens, it is iterative, in response to something concrete. The bottleneck isn't just thinking ahead, it's the judgment call when you see the result, its the walking back, as well as thinking forward.