In the past, I only upgraded twice—once because my old iPhone got bricked by an update, and once because I wanted Apple Pay (security reasons).
In the future, I would upgrade for one or multiple of the following reasons (depending):
- smaller screen (phones are too big these days for my liking, even the smallest iPhones)
- better camera (I also voted for night mode but I think that is part of camera).
- other:
- more storage
- bring back Touch ID
- bring back a thick chin or forehead bezel
- foreword facing speakers
- Mac MagSafe-like charging/data-syncing port
- some feature I didn’t know I needed like Apple Pay
- something happens to my current iPhone