The term "Retina" is all about context. The current panel in the 6s is not enough for me personally. 1334 X 750 in a 4.7" display equals 325.61 pixels per square inch. Now, in order for the display to appear perfectly sharp, I have to hold my iPhone 6s at least 10.5" away at ALL times or I no longer have a "Retina" display. I am one who likes to read closer than 10.5" because the 4.7" display is quite small. The reason the 6s Plus seems so much better is because with a 1920 X 1080 panel in a 5.5" display we get 400.53 pixels per square inch. Now we gain an extra two inches in our viewing distance and the display will still stay perfect sharp. 8.6" to be exact on the 6s Plus. May not sound like a huge difference but, the effective resolution is even greater because we have 40% more viewing area attributed by the increase in display size. That's where the double whammy comes into play.
"Retina" in my mind is about having a display that will stay "Retina," meaning pixels will not be resolved up to 6" for me. The S7 Edge does just that with a 2K panel in a 5.5" display. The smaller the display size, the greater the need to increase the display and pixels per square inch because our effective resolution is less by virtue of a smaller display that forces the user to hold closer to their face.
It's also interesting to me that my iPad Air 2 looks sharper and crisper than my iPhone 6s, even though I have to hold my iPad at least 13.5" away. Well I've got more than double the screen area, so I really don't need to get near as close as I do with my iPhone 6s. The iPad therefore ends up looking much sharper at 264 pixels per square inch because we are displaying that on a 9.7" display. I just don't need to be as close.
1080P would be perfect for the 6s and they could honestly leave the 6s Plus where it's at. 2K would be nice to though but you have to get pretty darn close to that display to see the difference between a 2K panel and a 1080P one in a 5.5" display. Your eyes can only focus at a certain distance.
With this said, I love having a display that stays "Retina" at all viewing distances. The 6 Plus was the closest device I've had from Apple to do that. The S7 Edge I used for two weeks did that at all viewing angles no matter how close I was to the display. It was quite amazing to use.