Apple's hands are tied by decisions made long ago re display layout. Because elements are positioned absolutely and not via a layout manager, they can't go with an intemdiate resolution - they're tied to either 1024x768 or 2x that.
Cocoa Touch has a scale factor for handling retina displays, in the form of the UIView contentScaleFactor property. It is a float, so if there were the will, unusual scales could be handled, somehow, even with absolutely-positioned elements. The UI for my game is laid out based on a 320x480 screen, and yet we somehow manage to make it not look too horrendous on the full range of iOS devices. It's not impossible. Images would be more of an issue than the actual layout.
But the latest iOS has some kind of layout manager anyway. Perhaps the iOS device range is going to become even more fragmented in the future. I'm not sure that's a great idea from anybody's perspective, but as long as people keep buying the devices, developers will continue to suck it up.