I think you are looking at it wrong.
What happens is that as tech evolves, the perceptable graphical difference from one generation to the next is less noticeable by the human eye.
I imagine next gen will be an even harder sell.
It also begs the question, do we even need better graphics than this? Personally, i don't. Though i could do with better performance (60fps). That s what i'd like from next gen.