You are right. On the other hand when I saw the iPad mini it looked horrible compared to the Retina iPad so what happens to your argument? With the iMac there are no power constraints (as opposed to the laptop), but the sheer size of that panel makes it just a $ 5000 panel today. I guess Apple is waiting for the price to come down.
There comes a point where the eye can no longer discern individual pixels. At that point you have a valid "who needs it" moment. While new apps and processes will require more and more memory, the eye will not see any better. But yeah, I'll throttle that CC in a second if they rolled one out.