Colleague of mine is an ex-dev for Apple, and spoke to me a few years ago about the battery issues.
When batteries start to degrade, high performance app's won't run properly or not at all because the battery can't provide enough power for the requirements. They basically keep band-aiding it in software to account for poor batteries. And that's the reality, that as batteries age, they just don't perform as well and create a whole raft of problems. The fact that batteries are non-removable just adds to the whole e-waste issue, as I suspect a lot of issues of phones comes down to a bad battery and not the phone itself.