I don't think I've ever upgraded my phone while the previous one was still operable :)
That said, Apple is pretty good about making sure their phones fail on a predictable timeline (2 years? Oh, time for the battery to stop retaining charge, or the touchscreen to stop responding, etc).
That said, Apple is pretty good about making sure their phones fail on a predictable timeline (2 years? Oh, time for the battery to stop retaining charge, or the touchscreen to stop responding, etc).