Don't think for a second that they wouldn't charge 30% once they get enough market power. In fact, in the Apple lawsuit, it was revealed that the Epic Game Store was operating on a $300-$400m loss in order to acquire marketshare. Epic has to increase commission fees in order to break even and make a profit eventually. Epic isn't a non-profit.
Here's a preview how they might treat customers of their own app store if they have market power:
Epic Games, the company responsible for Fortnite, agreed to pay $245 million to settle FTC allegations related to in-game purchases. According to the FTC, the company charged parents and gamers of all ages for unwanted items and locked the accounts of customers who disputed wrongful charges with their credit card companies.
https://www.ftc.gov/enforcement/refunds/fortnite-refunds