One theory for the iPhone mini's lack of success is people currently equate an expensive phone as a somewhat larger device (but not huge). The iPhone mini looks a bit like an old iPhone 5 from a distance. Paying a premium for something that looks like what everyone already has two or three of in their junk drawer is a tough trade-off for many people. The longer battery life then becomes the rational anchor.
In any case, I certainly hope they make a smaller devices again in 4-6 years.