I think the free market should determine this, via competition. There needs to be options besides the iOS App Store. Epic says they can profitably operate a store only taking 12%, and they back it up by actually doing so on Mac, Windows, and Android.
They can't demonstrate it on iOS or game consoles - I'm not sure whether game consoles should be required to have third party stores or not. There needs to be a way to enforce DRM, but I think that can be done by having strict rules on third party stores, rather than banning them entirely.