Go use Windows 8, or Windows Phone, on a lower-resolution device. It’s not immediately obvious that the display has less pixels. Why?
Simple — the UI masks that fact by using clean lines and right angles, instead of curves and skeumorphic designs. iOS, at least thus far in its life cycle, has always been closer to the latter than the previous option. With its round-rect icons and exceedingly realistic textures, iOS obviously needed extremely high-resolution displays to allow the UI elements to come in to their own.
With iOS 7, that may not be quite as important – if the rumors prove to be correct, we could see an interface that would have more in common with Windows 8 than with the skeumorphic design of iOS as we know it today.
That isn’t to say that high-resolution displays are useless in “flat” operating systems. For instance, text will always look far better on displays with higher pixel densities. Regardless, if the next-generation iPad mini ships without a Retina display, it may not be quite the end of the world that most would expect it to be.