When you see ESPN and Comcast argue over showing the World Series, don't buy the argument that one of them is "trying to prevent loyal customers from being able to see their favorite game", or when your local hospital group withdraws from your employer health plan that "the other side is trying to deprive you of consumer choice".
Each side is wanting to make a share of the money, and they're disagreeing over the price (or often the royalties share). No one is entitled to any particular provision of service in any of these cases. Except in regulated industries there's no law governing the "fair" share that someone has to offer, or someone has to accept. And in most cases, each side could choose to compromise what it's asking for with no damage to its model of business (not talking about $, just the principles they claim).
Spotify wants a lower $ charge. Apple owns the platform and controls that access and $ charge. That's it.
This is a private contract dispute and only of interest because you care about listening to music. There's no public right to have a music app be charged a certain amount that's called "fair". Spotify could charge nothing to consumers, and be charged nothing by Apple. It's their choice. It's Apple's choice.
Don't be fooled into "principles" when there's money involved.