Not buying Apple's excuse.
Why is it that iPhone 5S and all the ones before never have this problem of just turning off? Something else is going on...
Well, newer phones can draw much more power than existing phones could on a single cycle. And also, models before the 6 certainly turned off randomly all the time with age - dying around 20% or high on an old battery was so common it became a joke around my circles. Not saying it is that necessarily of course, but there's a lot of variables to juggle.