I think the current system is too much in Apple's favor, but I imagine they do have to spend a decent chunk of change curating the app store, validating submissions, hosting the apps for download, and maintaining the security infrastructure for developers to sign their apps and users to have confidence in them. This is all value added for developers.

That is all in service of getting people to buy their phones. Those are no value added to the developer, they are restrictions on the developer so Apple can make statements about its ecosystem.

The emails from the testimony indicate they wanted only a $1 billion run rate. They have a $4+ billion run rate. Do we really think it costs them $1-4 billion to run the apps store? I'd be shocked it cost even $100 million a year to run it.

The excuse that the 30% cut exists to pay for the costs of running the store seem absurd.

If a grocery store allows you to order groceries online, but they make it into an app instead of a website, will Apple take a 30% cut on the groceries too?

Nope. Retail goods are excempt already and can use their own payment processors too. Big obvious example is the Amazon app where you can buy anything so long as it isn't digital content using Amazon's payment system.