Very good question. For some reason having differentiation in the smartphone space is known as "Fragmentation" and bad.
Personally, I'd be very happy with Apple bringing out a wider mix of phones, but they seem dead against it.
Edit: Just seen your comment at the bottom of the page - I totally agree - Apple's lack of different options is probably going to go badly for them in the long run.
To make development easy and to make it easy create good user experiences across all iOS devices. None of their iOS devices have multiple versions in a single product cycle.
Personally, I'd be very happy with Apple bringing out a wider mix of phones, but they seem dead against it.
Edit: Just seen your comment at the bottom of the page - I totally agree - Apple's lack of different options is probably going to go badly for them in the long run.