Well I got the 6s because I had an iPhone 4 that I had bought the month it came out. Still worked fine, good battery even, but it had just gotten to the point it was time. Needing to upgrade the OS was a key deciding factor.
Given the Samsungs and Motorollas that my wife and family have burned through in the same span, I've found the reliability on the Apple offerings quite satisfactory. Still its not a bad idea, even careful users can have a bad day.