[zefhous]

The intermediate resolution of 3840x2400 exists solely as a tiny step in the rescaling algorithm, a step which in itself does not add any information that wasn't there in the original 1920x1200 image.

The thing you're missing is that this step does add information. OS X will render retina graphics for the UI instead of the traditional graphics, then those higher-resolution graphics are downsized. So, instead of an 18x18 icon, it will use a more detailed 36x36 icon, then downscale that. In the case of apps that haven't been updated for retina displays you are correct — certain assets will be upscaled. But the operating system and many core apps are ready for retina display.

Nobody is saying it's a perfect solution, but it's much better than just upscaling. The point is that at these densities downscaling can look pretty good — supposedly better than the native resolutions emulated by the downscaling.