The Mini is not mainstream. Most people want a big phone, basically big screens. But some of us also value size and weight. Never going to be the majority, so the Mini cannot sell in big numbers and goes on the chopping block relative to ROI with respect to its development cost. The cheapest iPhone has a entirely different purpose, it is to draw new people to the iPhone ecosystem with a lower price, the idea being that those new people will want to stay in this ecosystem and generate upsell to the expensive iPhones. I assume Apple has data on how many inexpensive iPhone users stayed in the system and upgraded. The Mini instead takes sales away from the expensive iPhones, if a Mini wasn't available, the Mini-buyer will opt for the next smallest screen available (presumably at a higher price).