Apple devices last a long time and get software updates for many years. You really don't need to upgrade unless either (A) the new device has features you want,

Apple devices last a long time and get software updates for many years. You really don't need to upgrade unless either (A) the new device has features you want, or (B) your current device stops getting software updates, or potentially (C) you have some new use case that would be more performant on a newer device.