Given publicly reported financials for Apples services gross margins imply a break even around 10%, I doubt any successful App Store offering similar services could survive on less than 15%.
Not for in-app purchases they don't, which is why I specifically called that one out. The game provides all the UX there to find & purchase the items, the game "delivers" the resulting purchased goods, etc... All the app store does in that case is facilitate payment. No different than, say, Square.
One of the biggest developer benefits is Apples app reviews process. While not perfect for devs, it helps ensure customers feel safe buying my apps free of the risk of viruses, malware, and fraud ware. That sells more of my apps.
If you don’t understand these things, you don’t understand your customers.