I get it, it's too small to notice any difference. The issue is that the phone is down converting a 4K image and video to 1080p. So you have captured this great 4K still or video and you play it back on a 1080p screen. When you zoom in, it will not do it justice. I get the battery savings and all, not sure if the iPhone can use a scaling solution similar to the retina MPR and MB retina. Thoughts.
It's similar to having this great 4K BluRay title and you are viewing it on a 720p or 1080p screen around the ~50 inch size. It's nonsensical.
If you have 20/20 vision and hold phone about 1 foot away, the maximum dpi needed is about 300. Any DPI above that is unresolved and not beneficial. It could be 100,000K and would look no different. That is why we can't see the atoms that things are made of.