When the average subscription to just about any online service is 3.3 months, the 15% after 12 months is simply PR. Nothing more. Apple crunched their numbers and knows exactly what they are doing. What's more amazing is that everyone, including you, has fallen for it.
Considering most cancelled subscriptions are cancelled within a trial period, the average subscription duration is heavily weighted down. Your point would be something worth considering if you had an average that excluded all cancellations within, say, the first month.
The idea that subscriptions are the main issue is just fluff. Under 1% of Spotify app users subscribe, for the rest all the add revenue goes to Spotify. Apple distributes the app and gets nothing.
As someone else pointed out in a different comment, Apple benefits from the app ecosystem even when they aren't getting cash for an app so they aren't getting nothing. Apple also has their own ad network so they can profit directly on iOS apps that are ad supported [0].
