Creating a device that every person needs to replace after a few years is a very sound business model. The issue is how many years you generally need to replace it. Apple used to be able to convince folks to replace their phone every two years. Now a two year old phone is not bad at all, and many people are moving to a three year cycle or even longer. This is significant, if 50% of folks move from replacing their phones every 2 years to 3, that's a 16% drop in sales.
Apple's real challenge is to keep innovating to convince folks to keep to shorter phone life-spans. However, as Moore's law keeps it's pace towards the graveyard, that will be be harder and harder to do.