For those who have significant sums invested in the App Store, they are effectively dominant as there is no way to switch app licenses to a different app store.

Sure, but the rules are _reasonably_ unambiguous and unchanging. The 30% cut has been in place since day one. If Spotify didn't like it, they had the choice to focus their attentions exclusively on other platforms. That wasn't the case with Windows+IE, when effectively there were no other platforms of any size.

And worth noting that Spotify grows by millions of paying subscribers every quarter without the app store. They're currently choosing to focus exclusively on other channels rather than try to pay the 30% and… doing better than Apple Music…