You are ignoring the world of products that Apple prevents using their control over the App Store, which they absolutely do use to exclude or hobble competitors

You are ignoring the world of products that Apple prevents using their control over the App Store, which they absolutely do use to exclude or hobble competitors and stifle controversy (such as how they have pulled multiple apps that were commentary about smartphones being made in sweatshops). Apple is already past the "live long enough to become the villain" half of "you either die a hero or". The anticompetitive effects of having a duopoly--both of whom being interested in complete vertical integration--in control over all software distribution is devastating, and it isn't just some improbable/vague second order effect (which is also a ridiculous assertion even for price improvements: there are tons of products that are having to charge more only because of Apple... mine is one of them! it really depends on the app's sales mechanism whether 30% is fair or not, and this is coming from someone who has run a successful competitor to Apple on their own platform, despite the users having to be rabid for it due to how many hurdles they had to jump... before Apple finally "won" :/).