Usually I have upgraded, but I currently have no intention of buying an iPhone 7 or later, unless they fix something!
For me it's a double whammy. I don't like the lack of headphone jack, and whereas I feel like I could manage grumpily ... for me it kills the excitement of buying the new device, and I think that is important. (My standard listening headphones are Etymotic ER-4Ps; there is no way I am going to downgrade to AirPods).
But the bigger part of the whammy is iOS. iOS is completely terrible at this point. I just can't consistently control the phone. A large percentage of taps or swipes do things I did not intend (how many I'm not sure -- 20%? 33%?) It's just completely crazy. They need to get rid of 3D touch, get rid of double-tap one-hand accessibility mode whatever it's called, get rid of weird swipes from the edge, fix the horrible inconsistencies in the way autocorrect works (or, please, offer a system that just underlines words-thought-to-be-wrong without changing them, and let me tap on them to change them, or use the current autocorrect system but let me tap on a word to un-"correct" it, the fact that the current system just changes what I typed and gives me no recourse to fix it apart from laborious deletions and re-typings, which I often have to do 2 or 3 times, is just haughty and offensive)... and in the meantime, might as well redesign the rest of the UI. Because right now the phone is not a joy to use, it's a constant exercise in frustration. I haven't felt good about using iOS since sometime back around iOS5 or 6.
So it's no mystery to me why sales might be slowing ... I don't want a new one if it's going to continue the downward trend.