The iPhone 6 still works pretty well. If not for carriers offering the iPhone 7 for free after iP6 trade in, I'd still be using one.
Smartphones are pretty mature now. Also, prevalence of inexpensive prepaid plans and the switch from subsidized 2-year contracts to equipment installment plans means more transparency in terms of delineation between plan and device pricing. We keep our devices longer now that we stop paying "subsidy" after the 20/24 month mark.
You can attribute that to Apple and their unhealthy obession with the iPhone 4/4s. Rather than move the 5 down when they launched the 5s like they did the 4s when they launched the 5, they repackaged the 5 with slightly better internals and a plastic case (iPhone 5c). At the same time, they did not discontinue the 4s.
But they most certainly cut off the iPhone 5. After only one year of sales.
Yeah, having bought a 5 on launch day as my very first full on Apple iPhone purchase - I'm still bitter about that.