Sorry, but this isn't a fad. Plain and simple.
Thanks for all the supporting evidence of your assertion.
I am barely five feet tall and I've got tiny hands. I LOVE my ginormous iPhone 6s Plus.
Different strokes, different folks.
I personally have very big hands, but I have no interest in using a bigger iPhone 6, much less a Plus. I slip my iPhone in my front pants pocket without a case (I have been since the original iPhone in 2007), and as such I want it as small, slim, and slick as possible to facilitate easy storage, and withdrawal. When I stand I don't want an unnecessarily large device protruding from my pants leg, nor do I want it changing the lay of the fabric, or restricting my movement, or uncomfortable when I sit. I have found all four of those to be a problem with the iPhone 6.
I have iPads, and MacBooks with AirPlay at home when I need a bigger screen. Using the iPhone for that purpose is pointless, even with the 6 Plus. When I'm mobile, I don't need a screen any bigger than the 5s to glance at a message, or get quick directions. I'm certainly not surfing the web when I'm out and about either. And if I did need a bigger screen for some reason, any location with an TV will accommodate that need via Airplay ... something that may become more ubiquitous over the next few years. Who knows, I may even be able to use AirPlay to take over any Apple display at some point, including the iPhone's.
So, I'm holding onto the 5S until the iPhone 7. If there's still no 4" screen model with Pay, then I will reluctantly upgrade to the next smallest size. Depending on the features of the 7, I might just hold out to the 7s.
The way I see it though, as Siri improves for use with devices like the Watch, I can imagine a trend where the phones shrink as there will be less reliance on the screens, and more reliance on audio relay of information. The Watch is clearly headed toward replacing the iPhone completely, and in the future to the extent someone needs a display, they will carry a dumb terminal that tethers to the Watch, instead of the other way around as it is now. Perhaps a foldable, or flexible display, even. Because I can't imagine why anyone would actually want to carry around a bigger device for any other reason than the display. Replace the display with projectors, or flexible membranes, or whatever, and the goal may be again to make the phone as small as possible. Prior to the advent of the Apple smartphone, flip phones were on a race to be the smallest possible. Why? Because given the choice, people don't like to carry more than they have to. As iPhone internals continue to shrink and battery life improves, the only reason to maintain a 6 or 6 Plus size will be the screen, and that's just wasteful for those who don't ned it.