Nobody is going to not put their app on the apple app store. Look at steam for example. Almost every game on steam can be purchased somewhere else, like icth.io or the game's website. 30% is industry standard among monopolistic players (Google play store, Apple app store, Steam). There are many other storefronts that take less of a cut, and don't require IAP to go through a specific payment system that the platform also owns. FDroid, MS Store, Epic Store, Galaxy Store, GOG, the list goes on.