Then what's the yearly $100 developer fee for? Weren't they saying that's for supporting the App Store and the core technologies they're suddenly charging again for? The only thing Apple has to host is the app. The rest is handled by Spotify. And as far as I know, Spotify isn't even built on Apple tools for iOS specifically. It's developed with React Native.
The "you get access to all Apple users" argument is so stupid. Developers is what make the iPhone. I'm not buying an iPhone because it has an app called "App Store", I'm buying it because it can get the apps I need and I don't care where from that would happen and how they call it.
Makes sense for indie developers that they might get some exposure thanks to Apple. But that's not at all promised once you sign up. Actually they don't even do it at all. The apps Apple pushes on the home page of the app store are the same as they were 4+ years ago. Clash of Clans and the same 5 Arcade games, a few photo editing apps that are extremely popular already, Duolingo and similar that everybody and their grandma has heard of. These companies have huge marketing departments already and don't need Apple to promote them, yet they are taking up those spaces. Imagine if Facebook was not available on the iPhone, their market share in Europe would certainly be hurt by that. Never have I seen Apple promoting an indie dev's app on the store. Most apps I have, I found out about them through developers' self-promotion. When I first got an iPhone there were several times where I was like "Ok let's check out what's new on the App Store" only to be met with the same 5 apps. Apple does not acknowledge devs in the beginning and unless they gain some serious mainstream traction, they never will. I doubt they even check with the amount of scam apps that pass through their "rigorous inspections" so they can put the false idea that it's them who helped you get there to rest.
Clearly, they think that right now the risk of developers abandoning the platform is worth it because they have the heavier hand and switching to another service is easier than making a new hardware purchase should somebody decide to leave the App Store. But all it takes for such a house of cards to crumble is a few major players to pull out in a sign of protest. We've seen this type of digital protests happen in a day.