The closer a product is to perfection the less it's going to need to be changed. I'm not saying the iPhone is perfect, it could stand some tweaking, but isn't it just possible they got it pretty close to right from the beginning and thus all that's really needed are changes of an evolutionary, rather than revolutionary nature, a continued process of refinement rather than a reinvented phone every 2 years? Change for the sake of change is, well, stupid.
Hypothetically, even if apple perfected the design and the phone couldn't be any better, every 2 years people would want it to change, even if it means a lesser functioning device simply because people always seem to get bored and want something new and different, sad but true.