Should I upgrade?
Some reasons why you might consider it:
Battery life: But you could also just get the battery replaced.
Camera quality: Some minor differences between phones which are 2-3 generations apart.
Cos of a new phone: Is it worth what you gain for how much money you pay?
Screen resolution: You might notice a great improvement in some of the newer models. But if you're satisfied with what you have and want to save some money, is it that big of a deal?
Compatibility: Does your phone take the latest iOS? Are there apps you are no longer able to use?
Peer pressure: Do you want to look cool having the latest iPhone? Or do you want to look frugal still limping around with an iPhone 4S?
Consider that, like a vehicle, many parts on an iPhone can be repaired and keep your device useful for longer into the future. The following are among parts that your average quality iPhone repair shop can pretty easily replace: broken screens, charger ports, cameras and dying batteries.