Fortnite grosses hundreds of millions of dollars on the App Store [1]; every dollar spent is a cut Apple counts toward its $10B+ annual gaming revenue numbers [2]. It is not an understatement to say that Fortnite, as a single application, is responsible for single-digit percentages of Apple's gaming revenue (which, while small as a whole, is a TON of money).
This move hurt Apple, full stop. It will likely cost them somewhere in the range of $50-$200M per year. You can quote me Apple's annual revenue, but I know what it is. Fortnite's contribution to it is small, but its probably far, far larger than most people here realize.
Of course, it hurts Epic more on the short term. But, long term, maybe Fortnite gets to come back at a lower rate; maybe they'll get to use their own payment processor; maybe the courts will actually work, and they'll force Apple to allow competing storefronts, which would enable the Epic Games Store to release on iOS, earning huge revenue for Epic.
And what's more: Epic's bread and butter has always been Unreal Engine, which is charged at a rate of 5% of a game's revenue (above $1M I believe, below that its free). Unreal is absolutely used for iOS game development. If Epic can win even a lower rate for all game devs, it amplifies their iOS earnings on Unreal; more money in the devs pockets means more money in Epic's pocket.
Epic's warchest is massive; its not just Fortnite, but also money from Tencent. They have the support of their massive community, including impressionable adolescents. They picked a time just weeks after Tim Cook was torn apart by Congress for allegations of antitrust. They're joining the ranks of Microsoft, Facebook, Google, and every other company that Apple has screwed over with their policies. They can fight this out, and its hard to say what the exact outcome will be, but whatever it is, Apple will not like it. Apple is on the wrong side of history.
[1] https://www.musicbusinessworldwide.com/fortnite-hits-1bn-in-...
[2] https://www.cultofmac.com/632642/apple-worlds-fourth-largest...