OLED Improvement, thinner and more energy efficient, along with all the improvement from current iPhone 14 Pro.
Modem and CPU efficiency improvement from Shrinking Node.
Thinner iPhone, Sub 6mm from its current 7.8mm.
Battery Capacity improvement, Super Fast Charging, Battery Cycle ( Which Apple has kept it just above 2 years under normal usage ).
All while staying within the current 170g weight if not lower.
That is a lot of iteration, and enough work for at least 5 version of iPhone.
For a lot of people, Phones has reach a good enough point and buying another one is simple because of MNO contract expiring or Battery degrading they are buying a new one.