Look at more practical example.
Steam, is so big that other publishers said, no we won't put our games in your store anymore. We don't want to pay you 30%, we will make our own stores. Few years later guess where they came crawling back.
I mean really big publishers like EA and Ubisoft, on Windows where you can have a store at the snap of your fingers, you don't need Steam. But because Steam is so big and all people buy from Steam, you need to sell there.
In case of Apple it will be even worse, who will dare to not publish in their App Store and leave 99% revenue on the table? Yeah.