Two-sided marketplace can charge whatever it wants, it's their marketplace. They need to charge whatever they feel is right to make it worthwhile and provide enough value to both sides so that the marketplace can exist and grow.
Apple released their marketplace over a decade ago with 30% fee.
Developers ran away from Symbian, webOS, Windows Mobile, Windows Phone and RIM to Android and iOS, thus proving that the charge is fair and the marketplace provides the right value for the cost to be part of it.
RIM, Nokia, Palm and others tried to do something similar, burned billions, but the market (via consumers and developers) has spoken, and those companies are now history and a part of business curriculum how innovation and changing the status quo can disrupt existing markets.
A decade later, after playing part in allowing Google/Apple duopoly to exist, developers now feel they have a basic human right to have access to a marketplace of billions consumers for free.
It doesn't have to be a "cut". They could simply charge developers to host in their store. Because they started with this model doesn't mean it's the de-facto correct or moral option.
AWS doesn't charge for servers based on how much a company makes using their resources, they charge for the resources used. Why should Apple be any different?