Below are what each other iPhone I've owned lacked or left more to be desired that compelled me to upgrade
iPhone 3G: Camera, processor, screen clarity, camera flash which I frequently use as a flashlight, design
iPhone 4: LTE, processor, design, speaker
iPhone 5: RAM, Larger screen, Touch ID, camera, design
iPhone 6 Plus: Nothing really right now. A nice to have would be more RAM but it's not crucial enough where I would buy another phone just for that. Everything else has been pretty much what I have wanted in a phone.
Of course I'll upgrade in the future but coming on two years in I'm still largely satisfied. Market maturity.