While things are undoubtedly more complex for designers and developers than when there was just a standard 3.5" 320×480 163 ppi screen, a number of those layout

While things are undoubtedly more complex for designers and developers than when there was just a standard 3.5" 320×480 163 ppi screen, a number of those layouts are transitionary. Just as developers slowly phase out support for older iOS versions (and by extension older devices), within a year or two it'll be possible to maintain only two layouts for the iPhone (retina 3.5" and 4"), two for the iPad (standard, retina) and one for the presumed iPad mini. In time they'll be able to abandon both the 3.5" retina and standard iPad layouts and revert back to just one iPhone layout, one iPad layout and one iPad mini layout (assuming that's a real product) as the older models become obsolete. It's not ideal, but it's still a relatively simple set of layouts that allow the 'pixel-perfect' method to continue working well.