A mobile GPU is about $50-100. I wouldn't call that "very expensive." In your case, you propose adding a GPU that is used only when it is plugged in. That seems kind of pointless for a notebook that is likely to be used unplugged quite a lot. The rMBP isn't designed to be a desktop replacement or gamer PC.
Would it have been nice if Apple included a discrete GPU and been able to keep the size and battery life the same? Sure. But they didn't, and my guess is that they couldn't keep the size and battery life the same if they had tried.
No laptop can use a dGPU for more than a couple of hours of heavy use on normal batteries that come in a MBP
This just shows me that you have never used or owned a laptop with a dGPU. This is why you are arguing your point.
You also have no idea how much nvidia charges for a 650M GT and DDR5 prices.