I see zero reason to upgrade my 5S. I just have not seen a compelling selling point for the later models compared to all the major overhauls phones like the 4 and 5 represented.

There's the form factor I guess, but everything just feels incremental or like basic maintenance right now.

A more realistic reason is probably saturation, a little like Facebook's user base. They're going to pursue new market rather than get people to replace their current phones from now on, I imagine.