The other factor, besides the two that you mentioned, would be that smartphones are a “mature” product category and new models are not introducing significant improvements compared to last year’s. I still remember when I upgraded from an old Symbian-based Nokia to my first “modern” smartphone (an iPhone 3GS): it was a big change! My latest update (from an Android Moto G 3rd gen to an iPhone 7 Plus) was just an incremental improvement.
