My last two upgrades were because of improved battery life and a larger screen. 6S -> 8 Plus -> iPhone 12 Pro Max.
At the rate that Apple is supporting iPhones - the 2015 iPhone 6s is still supported - I can’t see anything that would make me upgrade in the foreseeable future unless they move to USB-C.
At the rate that Apple is supporting iPhones - the 2015 iPhone 6s is still supported - I can’t see anything that would make me upgrade in the foreseeable future unless they move to USB-C.