It depends on what you mean by “cell phone”, and no, we surely didn’t hit the peak in 2015. We’ve had tremendous improvements in cellular technology, data speeds, camera and many other features. If you include Android in this, privacy and good app permissions control was almost non-existent on it in 2015, and has only improved at a better pace in the years after it (incrementally).
It's just like how buying a shiny new car is pointless for most of us: old cars get us from home to the grocery store and back just as fast, they have airbags and crash safety testing, etc. Older cars are also less likely to have broken software updates than newer "smart cars", too. In rural areas farmers pay premium prices for 40+ year old tractors because they don't have the anti-right-to-repair electronic doodads that ensure only dealers can service the thing.
Sure newer phones are better, in particular low light cameras, but most apps outside of latest games still run fine on the S6.