Many iPhone users keep their phones more than two years, their device continues to perform, receives iOS updates the day the new iOS is released, not any reason for upgrading. My first iPhone was 3GS, kept using it even after it was unable to accept updates. Have my present iPhone going on four years, same with my iPad Air2, both perform as new; chasing hardware is not a good use of my money.
In my life I do not upgrade my refridgerator, washer and dryer, dishwasher, power tools, car, home, stove, and the like every two years. Other than bragging rights for less than one year, there is nothing gained upgrading the hardware. It is the software that is worth upgrading, security updates, new whistles and bells, and the like. Yet most Android phones do not receive the latest OS. Those that do it is many months to over a year after the release of that OS. Some Android vendors are crass enough to say “you want the latest Android OS, purchase a new phone”.
When my iPhone gives up the ghost, that is when I will purchase a new phone; not before. As I stated, mine is going on four years, has three-D touch, big screen, is fast, will receive iOS 12.