Another factor is the constrained supply of iPhone 6s.
But no matter how you cut it, the longer a platform stays the lower new version adoption rates will become.
There are a ton of iOS devices that cannot run iOS 8 because they are not built to handle it. These devices will never go away. When we get to the next-gen, the number of iOS devices that cannot run iOS 9 will be increase as one more generation is dropped.
The number of generations that are officially...