iOS adoption rates

I wonder to what extent iOS adoption rates are affected by the longevity of iOS devices.  For example, if the rate is truly determined by looking at the number of unique devices that access the App Store, older devices that cannot support the most recent iOS release would drag down the reported adoption rates.

It would also be interesting to see official adoption rates by device (iPhone 6, iPhone 6S, etc.) or at least device type or vintage (iPad, iPod Touch, etc. or 2015 purchase).  There are unofficial trackers that do exactly this, but it would be nice to have more authoritative data.

Finally, it would be nice if the official page gave some detail on the methodology beyond being “measured by the App Store”.