Apple only one model is not enough to compete effectively

 

The market demand large screen and high resolution. Many are OK with iPhone5 long and narrow screens. However, many are also prefer 4-5" inch display. Next year most of flagship smart phones will have 1080P display. Apple's 640P display will be far behind with much higher price. 

Anyhow, Apple's will not be able to main its margin, $200 cost sell for $600 because HTC and Samsung are happy with $250 cost sell for $400.