If your battery only lasted half a day before, it was already a severely degraded battery. The update will put a lot of strain on the battery (iPhone does a lot of re-indexing and other stuff after the upgrade), which might might have caused the battery to deteriorate even more.

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.