Yeah, but it's not like Apple keeps all of the revenue. There are multiple parties bringing value to the App Store. Apple brings customers who are happy to pay

Yeah, but it's not like Apple keeps all of the revenue. There are multiple parties bringing value to the App Store. Apple brings customers who are happy to pay for stuff. App providers bring their apps and services. Revenue is consequently split. That Spotify is beholden to another party (record labels) is not really Apple's problem. It's Spotify's problem. And the problem is that they're not really bringing much value to the table. They don't own the content and they don't own the customers. They thing they provide is listening suggestions. Turns out that's not actually all that valuable. If it was valuable, they would be able to negotiate better terms with the labels, since the labels would be worried that customers would stay with Spotify despite not having access to the music from a given label. But customers don't actually give a shit about Spotify, they care about the music. And Spotify doesn't own the music.