I wonder if it's more of iPhones <6 being "good enough"... that the tangible new hardware features of 6 are not compelling enough to motivate some with iPhones <6 to take on a new contract and or lay out the full price to replace one off contract.
Personally, I see the bigger screens as a huge draw (pardon the play on words) for the masses, meaning I suspect many of those who are clinging to old models are not turned off by the bigger screens. Instead, I think it's just that unless ApplePay is a big draw, the <6 iPhone one already has will still run every app, still display the very same home screen, still make every call, still text just as well, still ________________ as well or close to it. If one doesn't burn for a bigger screen and/or ApplePay, they may see their <6 iPhone as "good enough" and await the 6s or 7.
It does seem inevitable that iPhone will eventually reach a point where the pull to upgrade will slow down as added utility (of new hardware features) becomes increasingly nichey. Maybe the crowd sticking with <6 iPhones are satisfied with their bundle of benefits "as is"? Or they just don't want a new contract or to lay out so much money for an unlocked one just so they can buy other things with Apple Pay and/or use modestly-larger screens?