More than half of iPhone users haven't upgraded to a 6 or 6S yet

To Apple enthusiasts, it's hard to imagine that someone might go more than a year or two without upgrading to the latest and greatest iPhone. With new plans from AT&T, Verizon, and Apple itself that make it easy (if perhaps not cost-effective) to trade in your iPhone for a new model every year, many gadget lovers are well-equipped to always have the newest iPhone. But many, many people -- a majority of iPhone owners, in fact -- do not upgrade every year, or even every 15 months, according to dat...