Right now you as a developer pay $X for access to their eco system, and on top of that you pay 15/30% of whatever payments they process for you.
You can say "well actually, that 30% isn't all for the payment processing" but I don't why it matters what Apple's intention behind the payment is.
Businesses rarely map charges directly into what is being paid for, because that doesn’t work in practice. Charges are made at a convenient point and then used to pay for the service as a whole. This is common practice across all kinds of business.
This matters because people are claiming they it is just for payment processing and then arguing that 30% is too much, which is clearly a dishonest position.