I think Apple leaves the number on the iPhone because carrier contracts make it a naturally cyclical purchase: When users come off contract, Apple wants them to know that there's a new phone available with a big +1 number.
Conversely, Apple wants you to buy a new iPad as soon as you want one, and not sit around waiting for the N+1 model. Spreading out purchases over the device lifecycle gives them better margins as manufacturing and components get cheaper.
Conversely, Apple wants you to buy a new iPad as soon as you want one, and not sit around waiting for the N+1 model. Spreading out purchases over the device lifecycle gives them better margins as manufacturing and components get cheaper.