Retina display support

When the image is a result of capturing the real world on a camera-sensor, then this is indeed true, and as you say, algorithms that do more funky things than pixel-doubling perform well.

But we don't know that, generally speaking. (though we can attempt to detect it) Lots of images are the result of computation, they might be output from a graphing-library, for example.

What works well for a photo of a face, doesn't always work well for output from gnuplot.