This could be disastrous to free apps and to new, small developers -- maybe all small developers. I suspect Apple invests a very significant amount of money into developer tools, developer support, creating great, new APIs for developers, developing Swift, documenting all those APIs, etc., etc., etc. They'll want/need to fund this from somewhere. It could be by charging a much higher fee to developers than the current $99. They might, also, quit providing Xcode for free. This will result in fewer apps, fewer people exploring and self-learning Swift, and a large challenge to free apps. Why should Apple give away developer tools to people who don't use their monetization system?
Or maybe they'll let others provide the tools at great expense -- like the days of CodeWarrior, ThinkC, ThinkPascal, etc...
Alternatively, they could raise the cost of hardware -- effectively moving that cost to users. That will limit sales of hardware and put Apple at a disadvantage as the smaller platform player in the cell phone and PC spaces.
As I read these developments and Epic's claims, I get frustrated with how much they think they should get for free from Apple -- the platform, developer tools, language development, support, API documentation, etc...... Skirting Apple getting a cut could very significantly hinder Apple's platforms. It could be disastrous for the platforms as we know them.
I, like a number of others here, favor the true increased security (though not impenetrable) walled-garden.