Pretty good reasoning, I think. Though, per update #2, I’m fairly surprised Apple will upgrade the iMac without a retina display. Why bother waiting then? Perhaps it’s a completely redesigned device? Still, non-retina would be disappointing in the post-Retina MacBook Pro world. On the other hand, it would likely be a 27” 5120x2880 retina, which sounds insane.
Ah, but think how preposterous a 2048x1536 iPad sounded a year ago.