The iPhone is not the iPad. The iPhone is not the iPod. The iPad is not the iPod. The iPod touch is not the iPod nano. The iPod nano is not the iPod shuffle. The iPad is not the iPad mini... Please stop thinking like Microsoft users with no creativity and no understanding of people in general.
The iPhone 4, 4S, and 5 are all iPhones. They provide different value at different price points (they are like differently spec-ed iPhones). It is my opinion that Apple will keep this strategy for the full-sized iPad. Also, Apple owns the equipment that produces these products. These machineries typically last 2 to 3 years, and Apple does not want to just throw them out when there is a new design. Please refer to Horace Dediu on this.
At the same time, the iPad mini is designed to target a different audience and/or different usage pattern. So it's like the iPod vs. iPod mini (back then). A different product line within the category.