Apple's successive retina devices have decreased in pixels per inch, but have increased in pixels per degree at their intended viewing distance as larger high-PPI devices were introduced. Of course, sometimes people use devices at distances other than those the manufacturer intended, and some people have better eyes than others.
The point is, current high-PPI screens haven't actually banished the pixel from human perception just yet. There are still gains to be made, but they aren't nearly as significant as quadrupling pixel counts over previous devices.
As I mentioned, the claim that the resolutions sold are higher than the human retina are disputed. See http://www.pcmag.com/article2/0,2817,2364871,00.asp
Apple's successive retina devices have decreased in pixels per inch, but have increased in pixels per degree at their intended viewing distance as larger high-PPI devices were introduced. Of course, sometimes people use devices at distances other than those the manufacturer intended, and some people have better eyes than others.
The point is, current high-PPI screens haven't actually banished the pixel from human perception just yet. There are still gains to be made, but they aren't nearly as significant as quadrupling pixel counts over previous devices.