The "correct" amount is the amount that everybody is happy to pay. If Epic doesn't want to pay 30%, then 30% is too high for them. That's OK -- nobody is forcing them to pay it.
The competition you're describing isn't the cost of running the app store; it's the cost of maintaining the entire Apple ecosystem, including iOS, new iPhone hardware, etc.
> Adding an extra option can't possibly hurt you.
Yes it can, and I just explained how. But let me try to spell it out.
1. New iOS "app store" starts up, but only charges 0.1%. This is possible because they don't write the OS or any of the infrastructure; they're a simple indexer
2. Everyone moves over to this app store because the fees are cheaper.
3. Apple's revenue drops precipitously
4. In order to maintain the OS, Apple either a) raises the prices of iPhones b) starts selling my information c) invests less in the iPhone and iOS.
All three of those possibilities hurt me; so yes, a new app store does hurt me.