The thing is, when you do 3D, your displaying the same image 2 times. So the system doesnt have to render 120 images, its rather, 60 images diplayed 2 times. The current explanation in the OP simply make it seem impossible for any good looking game which is not the case. But I surely agree it will get better on next gen system and it will be easier for developers to integrate it.

for every new technology, there will be ways to cheat the system, the game doesn't have to literally run at 120FPS at 1080P like power for 3D to work, you can mess with it in different ways to make it work, accuracy, on the other hand, will be the difficult part, how much can devs sink money and time into making sure everything looks right with 3D games?