Why not just replace the battery rather than get new phones? My iPhone 6's lightning port broke around the time iOS 11 appeared, so I used the insurance we had on it to get a replacement. The replacement was another iPhone 6, with a new battery.

That new battery is working fantastically well for me under iOS 11. I have gotten through 2 days straight of use without a recharge. No exaggeration. It happened to be relatively light use, but it wasn't in low-power mode, and it did everything I wanted it to do for 2 days.