You mean the resale or trade in value of the old phone? Well, the cost of battery replacement from Apple is $70 or less, so if the phone is that old, then I’d say that person has gotten a good amount of value out of it no matter what they do with it after that.@subjonas What if the cost of the battery exceeds the cost of the phone?
But that said, IF the phone still suffices for him/her other than the battery, and will conceivably continue to do so for a reasonable amount of time, then there is still financial sense in replacing the battery. It’s still $70 vs $700 (or more) for a new phone.
Also, there is still environmental benefit the longer we use our devices.