If Apple did this, they would essentially be admitting the whole "retina" sales pitch was a big fat lie. I don't buy this rumor at all, it makes no sense. 2048x1536 is already an excellent resolution for the screen size, I don't see the point to it.
I don't know whether this rumor is true or not. But technology will not stand still for what you think is sufficient. Like it or not, the iPad 10 will be leaps and bounds ahead of the iPad 5 in display, interface, and raw power. For that to happen, the iPad 6 has to be incrementally better than the iPad 5 and the iPad 7 has to be incrementally better than that.
That cool parallax effect when you tilt your iPad back and forth will one day happen when you move your head from side to side over the holographic display. But the filter over the screen that allows the illusion of 3D to work is going to have to have a lot more pixels to work with than the paltry "retina" display can provide.