Ultimately, as we approach the limits of human perception, you need bigger and bigger jumps in resolution to have a smaller and smaller improvements in PQ.
Jump from SD (c.480 lines) to 720 and that's an increase of 50%. 720 to 1080 is a further 50% increase. Because of the way these things work, a further 50% increase will result in an even smaller improvement.
The implication is that a resolution increase of a further 50% (up to 1620 lines) will see an even smaller improvement than 720 to 1080.
Sorry lads, the games up.
We all want it, don't we. We all want to believe that there's some improvement out there that will 'transform' our viewing experience; some magical, as yet undiscovered/undeveloped system. And yet logic dictates that it's impossible for equally impressive improvements to go on forever.
Well, here we are. Not perfection, but not far off, at least not as far as resolution is concerned. The biggest step forward would be some sort of non-gimmicky 3-D. But on a flat screen, with a flat picture, we're near the edge.