For the majority of apps, the App Store doesn't provide discovery. It provides a nearly frictionless delivery and purchase interface. That's worth something, but not 30% off the top. That's worth a premium on top of processing fees, like X cents per transaction and Y% of the transaction, where Y < 6, I think.
Edit: I should say, I don't think there's any way this is going to happen. It would take an anti-trust action to make any difference, but that's not likely to happen.
Look at profitability of Android vs iOS — almost universally iOS makes a far greater profit that more than offsets the commission. I wouldn’t want to buy an app outside of the App Store because frankly, I don’t trust most developers with my personal information nor do I trust them to not engage in practices that are contrary to my privacy or enagage in sloppy coding that might subject my device to security risks. As a consumer, the App Store is great.
There are lots of b2b apps that are fully sold on the Enterprise level and the app is simply an add on. It is very frustrating to have to work within the walked garden when this is the case.