Nobody says that Apple shouldn’t make anything from the App Store
However based on the business model they have by selling new iPhones every year & making a profit plus the huge amount of traffic they have using the App Store they could easily still make a profit on 12 to 15% IAP charges because of the huge volume of traffic they have.
Which raises the question - what’s your yardstick, and who decides what a “reasonable” profit margin for Apple ought to be?
Another example is Steam, and I am not sure what exactly they do to help promote the gaming market besides aggregate a customer base for developers to sell to, and the company is clearly very profitable. Perhaps you can enlighten me on what Steam does to justify their 30% cut as well?