i find the problem with the mini iphone is that the features of the bigger iphones cast a strong shadow over it. pairing this with people wanting to pay for phones based on screen size alone and it means the mini and other smaller phones don’t have the margin to include new features - essentially they become a one trick pony.
during the apple keynote the most time is usually spent showing all of the interesting things that can be done with the new tech - and that’s mostly stuff that doesn’t come on the mini.
as evidence of this: i feel the recent success of the 14” macbook pro is specifically because it packs essentially all capabilities of the 16” model into a compact unit - and with that the price difference between the two is around $200.