10% loss in battery capacity in two years is not too bad, theoretically. But in practice, I notice the battery power drops much faster during use now, as compared to the phone’s first two years. (I had not tracked previous consumption so I don’t have actual figures.) In fact, one of the biggest differences between my 6S Plus and my previous (smaller) iPhones was that I could use the phone without ever worrying about the battery. That and its bigger screen are what make it a Plus in my eyes. Sadly, that’s no longer true.
I have no other issues with my 6S Plus. If a battery change can restore the phone to its previous state, that would definitely improve the user experience. And this user would happily use the phone for another year. Must add that Apple’s price reduction is a deciding factor. Anyway, I will decide for sure when I visit the Apple dealer.