Still using an iPhone 5, and aside from the battery getting too old to hold a charge, I'm still happy with it.
I think there's at least three things going on here:
1) as stated, the US Dollar is getting too strong and exports suffer. Yes, it's complicated, but this is almost certainly a factor.
2) utility of the iPhone has plateaued. They add features, and processing capability, and such, but there aren't any new use cases driving me to upgrade. I can still make calls, send text and data, kind of browse the web, and play some games. Newer models mean I can do those things faster, but there's not much I can do with a new phone that I can't with an older one.
3) I have 3 products to upgrade, and don't need to keep them all at the latest and greatest. I have an iPhone, and iPad and a Watch. Each time I added a device, I gained utility, but now it's pretty much incremental, except, I hope, for the watch. So, when I ask myself whether I need a new phone, the question is why? I don't need a bigger screen, or faster games because I tend to use iPad for those purposes. Will 3D touch be more worth the money than updating my Watch to the second gen? Who knows, but I'm guessing not.
So, I'm probably part of the reason the iPhone sales are declining. If Apple releases a 128GB 4" phone, I'm in. If not, I'll probably see if I can just get the battery replaced in my current device and see what the new Watch has to offer.