When the iPhone 5 was launched, I remember a promo video featuring Ive, and he said something like "it was tempting to completely redesign the iPhone, but people have certain expectations for what iPhone should look like and feel like, and we want to respect that and don't want to throw a jarring new experience at them" (paraphrasing, obviously).
Look at every other Apple product line. Gradual evolution, not revolution.
Until people stop buying it, apple isnt going to do anything radical with the iphone. They know they will continue to be bought so they can continue with the slow and steady upgrade route that they have been doing for years.