Apple risked their resources and created the whole ecosystem before Spotify even existed. It's not like Apple put a gun to the heads of Spotify executives and forced them to use the App Store. It's the executives who reviewed Apple's terms and then decided that investing in Apple's ecosystem would be useful for Spotify.
How should Spotify reach iOS users? Through gimped PWAs? Through non-existent thrid-party app stores? Your argument makes it seem like Spotify has a choice when trying to reach a large part of the market. The don't.
I am just going to flip this argument. Hope this gets my message across eloquently.
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How should Apple make a return on its investment? Through apps such as Spotify, who don't want to pay a dime? Through Governments, who are actively trying to sabotage it? Your arguments make it seem like Apple has a choice when trying to monetise a large part of the market. They don't.
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I think you are taking the efforts it takes to build an App store user base as granted. Microsoft is spending billions to build one with nothing to show for it. Google managed to build something half decent and it continues to be envious of Apple.
App Store and iOS existed independently of Spotify. Spotify decided to invest into Apple's ecosystem, by their own conviction. Apple didn't force them. Spotify, as a behemoth, and a large part of their market, probably only exists due to Apple. Therefore, any arguments to the effect of "it is unfair to Spotify" are difficult to consider.
Your flipped argument doesn't work. Apple gets tons of money from... the people who buy the phones! The actual users! That they then consider these users to be a captive audience who can't be trusted to use the device they paid a lot of money for, is the questionable situation. Why can't these people engage in business with Spotify to use it on their device, that they paid for, without Apple coming back and asking for more money?
Apple risked their resources and created the whole ecosystem before Spotify even existed. It's not like Apple put a gun to the heads of Spotify executives and forced them to use the App Store. It's the executives who reviewed Apple's terms and then decided that investing in Apple's ecosystem would be useful for Spotify.