Spotify can’t afford the 30% “Apple Tax”, and and Apple knows that

For your second paragraph all I could think about was how now is the time for Linux desktop. Then I realised that non-Android Linux could be good on phones too (for the problems in your first paragraph), but for some reason it didn't seem to catch on.

I say this not as a hardcore Linux fan, but as a Windows user of nearly 30 years.

From the sound of it, the cost to acquire customers outside the app store ecosystem is significantly more expensive. Let's say it costs them x dollars. And gross profit per month is y dollars. The average customer stays for z months before churning. profit is thereby reduced by (y*z) - x. In the app store, x is somewhat reduced, but y is greatly reduced due to Apple's cut. At the same time, Apple has its own competing offering which it makes a first class citizen in the app store, violating policies it enforces on others. This further increases x, presumable to the point it doesn't make economic sense anymore. So they must then compete without the app store on an even more unlevel playing field where their x is much higher than Apple's x.

>If Spotify can't make money on the Apple store why not just charge whatever they need to, pretend it doesn't exist, and go for customers through different channels?

Because Apple's App Store and Google's Play Store are where everyone is.

When they advertise through other channels, a significant fraction of the users they get will turn out to be iOS users, who, regardless of what the CTA is, will respond by installing it on their phone through the Apple Store. They start out as free users. Apple then disallows any attempt to convert them into paid users via the app, and also via email, and also via pretty much any targeted channel. So Spotify would have to not only give up on the channel, but also mostly give up on that customer segment.