Apple's cost to maintain this ecosystem can't be anything close to the insane $50B in App Store revenue they hauled in last year.
If there were competing app stores available on iOS, then we could say they are really entitled to that profit. But we don't know how much of this profit reflects the value they brought versus the rent they were able to get due to gatekeeping the only bridge into iOS-land. So no, we can't for sure say they are entitled to the entirety of that profit.