I've had the iPhone 6S Plus for two years, and I'm so happy I didn't upgrade to the iPhone 7, 8, or X
I bought the iPhone 6S Plus when it came out in September 2015 for $800+ (with taxes), and I couldn't be happier that I saved $800+ in 2016 and 2017 by not upgrading to the iPhone 7 Plus, iPhone 8 Plus, or the iPhone X.
Don't get me wrong - Apple's newer iPhones are great and objectively better than the iPhone 6S Plus. They have faster processors and better cameras with dual lenses, which makes for beautiful photos. Those two features are arguably the main reasons someone should upgrade to a new smartphone.