My history, and the primary motivators that got me to upgrade:
2G (4GB): 2007 - 2010
Upgraded to a 3GS after three years to get iOS 4 and more storage.
3GS (32GB): 2010 - 2013
Upgraded to a 5 after three years to get a retina display.
5 (32GB): 2013 - 2016
Upgraded to an SE after three years to get a phone with more RAM.
SE (64GB): 2016 - present
Got a 7 to supplement the SE to use the SE on a second line as a dedicated school phone.
7 (128GB): 2017 - present
I currently have a 7 as my personal phone and an SE as my school phone for parents to call/text. I seem up upgrade every three years or so, which I suppose would make me due for an update soon. However, I honestly have no burning desire to upgrade either of them, as they both do everything I need of them! I suppose when they stop doing what I need, I'll upgrade, but for now I'm perfectly happy. They both run amazingly under iOS 13. I don't have a pressing reason to update like I did with any of my previous iPhones.
2G (4GB): 2007 - 2010
Upgraded to a 3GS after three years to get iOS 4 and more storage.
3GS (32GB): 2010 - 2013
Upgraded to a 5 after three years to get a retina display.
5 (32GB): 2013 - 2016
Upgraded to an SE after three years to get a phone with more RAM.
SE (64GB): 2016 - present
Got a 7 to supplement the SE to use the SE on a second line as a dedicated school phone.
7 (128GB): 2017 - present
I currently have a 7 as my personal phone and an SE as my school phone for parents to call/text. I seem up upgrade every three years or so, which I suppose would make me due for an update soon. However, I honestly have no burning desire to upgrade either of them, as they both do everything I need of them! I suppose when they stop doing what I need, I'll upgrade, but for now I'm perfectly happy. They both run amazingly under iOS 13. I don't have a pressing reason to update like I did with any of my previous iPhones.