Earlier quoted context omitted.
Your battery is probably degraded, and no software update will fix it. Lion batteries just don‘t last forever. You can get a replacement battery for your phone for €25. If you don’t want to do the replacement yourself, any 3rd party repair shop will replace it in 15 minutes. You can also go to an authorized service provider, but that will cost €100 and take much more time.
Half a day, while far from ideal, was long enough for my needs. However, as soon as I upgraded, and not a minute earlier, the battery problem manifested. Now, it could be a coincidence and it is indeed just a problem specific to the battery, but its at best suspicious - the timing is. I will replace the batter anyway, and I am getting a new iPhone next week, I just don’t appreciate being “forced” to upgrade -- conven…
And, as I said, you are not forced to upgrade. A battery replacement is cheap and quick and will solve all your problems and your phone will feel like new.
I also have the iPhone 6S, and I've also been having trouble with the battery. I've had to replace it twice. (First time Apple replaced the whole phone because of a manufacturing defect, second time I went to a 3rd party repair shop)
Sure, it would be nice if Apple products lasted forever without service. Unfortunately they don't. But servicing iPhones has become extremely cheap and extremely quick -- as long as you stay away from authorised service providers.