Apple would argue that 30% is for access to their customer base. Of course we know the bulk of App Store revenue comes from game IAP so there’s very little Apple has to do there in terms of development/R&D. And then of course the question is who is actually bringing the customer to the App Store. I haven’t downloaded a new app or made an in app purchase in a while but I certainly can’t remember the last time I did so because of Apple. Usually if I download an app it’s because I hear about it on a podcast or read about it on a tech site. Not because of any promotion from Apple or the App Store.
I'd go farther and say the App Store is pretty useless unless you already know the app you want.
It's filled with ads, game scams, and just plain fraudulent apps. 99% of people probably just download the same big 10 apps