I think you just have to treat the battery as a consumable item. I had Apple replace the battery on my 6S and my X after they reached around 82% capacity. It took 2-3 years which I feel is acceptable.. I'm not a power user and could easily get 2 days of use before charging so if someone has to recharge more than once a day, I can understand why the battery capacity would suffer.. As iOS matures, I think battery performance will continue to suffer until new battery tech can be implemented to satify the need for more power