EDIT: I just realized:
> running the App Store, marketing the apps
What marketing specifically? Last time I checked, I had to integrate third party ad experiences in order to market my app in their ecosystem.
> In such a hypothetical world, developers could potentially avoid the commission while benefitting from Apple's innovation and intellectual property free of charge.
> Apple argued from the very beginning that the 30% was its fee for running the App Store, marketing the apps, and storing and delivering the app bundles
With hindsight of the antitrust case in the EU this argument itself seems like it is in bad faith. Apples argument in the EU against antitrust was that there isn't one but five different app stores, conveniently for each device family.
So they are indeed aware that a commercial app developer would likely not be "benefitting from Apple's innovation and intellectual property free of charge", because they are likely not taking advantage of their entire catalogue of intellectual property (if there is five different app stores, for five different markets, as per Apples argument, surely there is different IP between those markets to run the app store in an effective manner).
In other words, as others have mentioned, Apple charges a 30% flat fee for "intellectual property" that developers may or may not take advantage of, as per their own argument overseas. I don't see how it's not antitrust when a company forces me to pay for services that I am not taking advantage off or ever had the intent of using, or can't use, due to technical limitations.
As a developer, to me it rather feels like Apple is trying to artificially bind me to invest into the app store as a platform due to this high fee, for example by developing an accompanying watch applet or a tablet version of my app, because the act of simply offering these on their devices seems to be part of their precious App Store IP. And you can't argue that pricing these add-on apps differently would make a difference for Apple, since they collect the 30% cut across income in their entire ecosystem either way.
It also doesn't make sense for Apple to tell me to go elsewhere (Android) to mitigate this, as I can't replicate the UX of their ecosystem on my own even if I tried (prime example would be the peripheral integration imo. You'll find creating a device that integrates as seamlessly as the Apple Watch fairly hard, not because Apple has some special proprietary integration IP secret sauce, but because they artificially lock down ways to interface with peripherals to secure market dominance).