While we can and should have a healthy debate on the actual percentage, the reality is that every 'store' makes a profit on what they sell... or they wouldn't stay in business for long. Products are bought at $x, marked up y% and sold to consumers at $z. Absolutely nothing is different here. In addition, every store has rules about what products it will or will not carry. Again, no difference.
Referring to this as an "Apple Tax" is at best disingenuous and at worst downright deceptive. It's Apple's cut for providing the infrastructure, payment processing and profit. It's not a 'tax'. Developers do not have to accept this. They just need to handle payment for purchases outside of the app / App Store. Customers, however, *like* doing this all-in-one. Yes, it's a chicken-and-egg situation.
Frankly, Apple is incredibly generous providing the infrastructure to distribute free apps without requiring any payment.
From my perspective, the challenge - and where Apple needs to be very careful - is the combination of store ownership with what could appear to be a censorship approach to deciding what products are sold. Because the Apple App Store is the *only* gateway into iOS, it needs to be as inclusive as possible while still enforcing minimum standards. We, the customers, expect Apple to require a generous layer of protections, ensuring that any app downloaded (purchased, free, or free+in-app) will not create a security gap, breach privacy, and will work well.
Spotify's assertion that they cannot connect Siri and HomePod to Spotify unless they also submit to the 30% overhead is a bit troubling though. If true - and I'm wiling to wait for more information - then Apple is indeed being at least somewhat anticompetitive. IMO, that should be the focus of Spotify's challenge... and what the EU investigates.