One thing I like about other manufacturers is they have a variety of form factors. Different sizes, shapes, QWERTY, slider, flip, etc.
After three years, some people will get sick of holding a slab of grease against their ear. You thought flip phones carried alot of germs, what about all the stuff touchscreen picks up from your every touch. At some point, people will get sick of waiting for technology to improve and will start to hate the form factor or carrier exclusives to AT&T.
iPhone depends on Apple's time. An annual span. When technology is moving faster than they are, they will get left in the dust. Just this year, Apple played catch-up just bumping the MP to 5, adding LED flash, front-facing camera, and a 1Ghz CPU. It was a big evolution to iPhone, but nothing revolutionary to the entire industry. Nothing game changing. It has rested its laurels from apps and a slick UI from three years ago.