As for whether humans “need” or “get used to” a Retina resolution, that way of thinking is backwards. The default state for humans is real world resolution (no pixels), or for text, the resolution of ink on paper (600dpi to 2400+ dpi). What we had to “get used to” was the unnaturally chunky, low resolution of displays from the first computers until now. Those Dark Ages are over and now we can enjoy computer displays that have the resolution they always should have. A 96 dpi monitor is not “normal,” it is as archaic as a dot matrix printer.
People arguing that hi-dpi is not needed just boggle my mind. The world is hi-dpi, all the way down to the Planck Length, the “pixels” of the universe. Why would you not want readable text? Why would you not want excellent graphics?
Status quo bias in action, I guess?