Apple engineers are not required to answer, and not even the very helpful site https://www.google.ca/search?q=battery+university+lithium+ion&ie=utf-8&oe=utf-8& gws_rd=cr&ei=_oBhWL7lKMymjwScoKLYDA
in two full years of cycles a single bad cycle can damage a Li-Ion battery, e.g. going running or cycling with the phone on a hot day (left in car), or very cold day (-35C), etc etc etc. For example as I never take my cell at work- cannot bring it in, am used to seeing my cells with hundreds of cycles but less than 20% loss. but yes, a new OS with new SMC controller software might look at some of these batteries and deem them as non functional. Happened to me on one iPhone 5, barely used, a new OS bricked the battery draining it very fast. Apple replaced the battery and the problem went away.
So yes, bad drain, temperature and constant background usage can shorten a battery life pretty fast. And some, as the Fedex airplane crash demonstrated, are just bad from day 0. Even if 1/10000 batteries were defective, that is ONE HUNDRED PER MILLION, and so on. As Apple sells million of iphones, anything higher that 1/10000 failures begins adding multifold to this issue. Think Samsung: NOT ALL batteries by that manufacturer exploded. It only took dozens but wiped out 17bn of Samsung capitalization. Int he Iraqi desers I always kept my iPhone off and insulated, and left it months later without any loss of life. Many others used their phones and they failed rapidly.
An iOS poorely managing the energy consumption will therefore expose battery flaws exponentially faster (happened to Microsoft Surface recently, just google it). So this iOS merely exposed the battery flaw. Because, if every single Apple iPhone 6 6s 7 was affected - or even a third, Apple would be shut down with repair requests and it would be a major PR disaster. The current issue, although mediatized, is also affecting only a fraction of users and Apple likely knows which ones *by batch and ser. number)., but may be digging its heels to admit responsibility and avoid a major recall. So they could spread out the troubleshooting over 1-2 years and putting the time wasting effort on you, the owners, with all these needless steps.
I recommend Apple Genius bar and changing the battery. Pay for it- you will have 1 yr warranty. If issue is resolved, we know what caused it. If not, it is the iPhone itself shorting. I know people running 5 5s and 6 6s ios 10.2 no issues. Personally I am staying with 9, but that was a rebellion against Apple's new cumbersome iOS 10 styling and poor ergonomics.
