Arguing against myself, Apple could be discontinuing the smaller models because they did market research and found that most buyers of smaller, cheaper devices could be converted to buyers of larger, more expensive devices if those smaller devices didn't exist. Auto manufacturers are doing just that, discontinuing or enlarging smaller light trucks in favor of larger models that are subject to less regulation and therefore can be designed and manufactured more cheaply and might offer even more profit.
If Apple has or had that strategy, then my assumptions are flawed because no matter how many mini iPhones they sell, they would still want to get rid of the line as long as most of those customers could be converted to full-size iPhone customers.