Here's a hint. It wouldn't even come close to taking that kind of money to run a worldwide distribution network. As mentioned software has been sold over the internet, across the world, for a lot longer that the App Store has been around.
Go check out the Mac App Store and see how it works out when Apple doesn't have the app sales/delivery monopoly. Once excited developers have left it or avoided it like the plague, and are selling outside of Apple's store just fine.
This is a great point. If Apple allowed other means of downloading and installing apps on iOS devices, would the App Store fall to a similar fate as the Mac App Store? I think that it would.
I am a developer with a published app on the App Store. The app makes money using a subscription model at $3.99 per month. Essentially, I pay Apple $1.20 per customer per month. For what, exactly? From a technical standpoint, not much. Hosting and serving the app binary code, and billing. Web servers are dirt cheap, and most software billing systems range from 5-10% of the transaction. Those two are the necessary components to develop software that generates revenue. Apple provides some other nice, albeit unnecessary, services such as app analytics (can be had for free). Advertising costs extra $$ in the App Store as well, so it's not like that's even included. But we can still advertise elsewhere and link the user to the App Store.
Personally, if there was a way for iOS users to install apps from outside the App Store, I would investigate if it would be cheaper to host and serve my app and implement a different billing system. I can guarantee that it would be cheaper than 30%, and I probably wouldn't use the App Store. And I wouldn't have to pay the annual $100 Apple Developer fee.
Apple has an app distribution monopoly on iOS through the App Store. You can't get apps any other way. Since Apple makes iOS and iOS devices, maybe this is ok. I'm not a lawyer so I'm not sure. However, the fact that you are forced to use Apple's in app billing system (your app will be rejected if you try to link to an external billing system) is a bit ludicrous. I guess the $100 Apple Developer fee isn't enough to cover Apple's cost of running the App Store.
For subscription models, Apple does reduce the 30% to 15% after a year. But a lot of apps use consumable purchases like buying extras in games, and those will always be 30%.