Right now, both Apple AND Spotify have an anti-competitive advantage due to the app store (with Apple Music getting a massive advantage, and Spotify getting a better deal than new competition entering the market).
Although a lot of streaming payment rates are set upstream of streaming services.
And all fees can be avoided by not taking payment via the App Store. A streaming service can even accept payment in the Safari browser in iOS via Apple Pay, and it would not cost them anything.
99% of musicians being paid small amounts has only to do with the population at large liking a very, very small percentage of music much more than the vast majority of music.
That was the crux of the case. Spotify was accepting payment through their web site in addition to taking payment via the App Store. And because they were processing their own payments, they could offer the same subscriptions for a lower price than on the App Store.
The issue was that Apple was not allowing them to inform App Store users that they could pay through a browser.