Here is the culprit: if you know that batteries tend to loose performance with time, why should you encourage or even «force» your customers to upgrade their devices?!
Yes, of course the security argument could be involved but Apple should have had the moral obligation to inform their customers that their devices would run more slowly before asking them to make the move.
The iPhone 6S’ battery performance has to be analyzed the same way Apple did with the MacBook Pro Retina w Touch Bar and it’s battery performance: rather than resolving the battery’s problem, Apple just took away the feature 🤷🏾♀️
By the way, by asking the customers to pay 29$ for a battery replacement (which costs less than 10$), Apple makes a smart move: making those customers part of the solution 😁