The question I've asked others is if they open it up, then 30% is fair? It seems to me that is what you're suggesting.
But yes, it would be fair for Apple to ask for 30% because others are free to offer the same service for less, and you as a consumer of that service have a choice between them - which will eventually force Apple to bring its commission down.
Of course, I think most money comes from IAPs, not app downloads. The issue then is whether Apple would be forced to open the payment APIs, or if apps would need to implement a separate solution.
For example if my bank implements the payment provider API, then let me use that. All the other apps need to care about is a payment request method which can be satisfied with whatever payment provider I have picked.
Apps would be able to ask the provider how much they’re gonna get after the payment processes etc, and decide if they want to reject it or not.
The problem is large companies bundle too much market power together. If you want to avoid paying App Store fees currently you have to pull off a Herculean effort no company in existence is capable of.