Excellent point. However, there is a threshold of the amount of data a CPU can handle before it bottlenecks a GPU, which is how CPU/GPU bottlenecking happens.That is true, however that part is independent (mostly) from rendering itself.
In short :
If you throw enough pixels/AA at GPU - you will always end up GPU b'necked, regardless of what CPU you are using.
Which is what you see when a ~3,6GHz Q6600-type CPU can almost maxed out GPU usage on GTX 1080 when playing GTA V (in some specific places).