I don't think I ever said those improvements wouldn't find their way into new iMacs.
Yes, the ultimate MSRP of an Apple computer (and most everything else) depends on a lot of things, but primarily cost and expected sales. My arguments only relate to Apple's costs. I'm sure they expect to sell a boat load of rMBPs, and so maybe the margins are smaller, compared to the old school MBPs. My only point is that adding a retina display to any computer -- keeping all else equal -- adds a lot to Apple's costs. You almost seem to imply that that's not the case.
Apple doesn't sell many desktops, even iMacs, compared to portables. Their margins are correspondingly higher, and starting prices are lower. They're also consumer oriented machines, so the primary end user is more price sensitive. Furthermore, the LCD panels are bigger; granted, the retina ppi will be smaller, but there will be greater numbers of pixels compared to rMBP. Therefore, if Apple wants to bring retina to existing iMacs, they will have to,
A) Cut costs, but there isn't much room since the iMacs already use cheaper desktop grade components (with the notable exception of the GPU).
B) Shrink margins, but do they really expect a commensurate increase in sales, when portables and iOS devices are becoming increasingly popular?
C) Wait until retina panel yields increase and the cost is more palatable.
I think the general consensus that a non-retina iMac is due this year, with the retina version appearing next year is because Apple is going with C.
