If EPIC opens a competing App Store and only charges 15% instead of 30% that could help other developers. The fact that they make money too doesn't negate the benefit to other developers.
Yet it seems that Epic completely ignores what truly benefits other developers: the 16% of apps that are paid, in some form, subsidize the 84% that are free. A free app gets the same tooling, same support, same push notifications, same cloud asset storage, same cloud syncing, etc. – the dev certainly benefits, and one would hope it's been downloaded and its users, maybe even some bit of society as a whole, are benefiting as well.
And pardon the digression that follows, but every alternative I've seen falls into one of, I think, three categories:
(1) Developer Program fees should cover the costs. I'm old enough to remember when this was indeed the business plan. Annual layout was easily an order of magnitude higher. Tools, documentation, tech notes, sample code, you name it... price tag on everything. And after shelling all that out, every step of actually getting the product into users' hands was still entirely up to you. No thanks.
For those apps that
are paid, hobby-level earners, the benefit of having all payment processing, all sales tax reporting and distribution automatically handled, is HUGE. Anybody who hasn't ever dealt with all that themself is likely seriously underestimating the actual bottom-line costs (processing fees, labor, time, stress). And farming such admin stuff out isn't cheap.
(2) Fees should be tiered by revenue. So the tiny percentage of wildly successful devs should pay significantly lower percentage? Let's just call this the "1% plan", wherein the richer you are, the faster you get even richer, by systemic design. Wow, no... pass.
(3) Umpteen varieties of opening things up and/or lowering fees. Newsflash: Apple has been selling Customer Experience since 1984. If the customer didn't find a compelling value proposition in what Apple provides, they simply wouldn't be a $2T company. Nor would they have gotten there if overall, despite its remaining imperfections, this weren't on balance a (company-devs-customers) win-win-win system.