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].
