The district court and the appeals court both agree that Apple is well within its rights to charge developers $99 initially and x% of revenues, under the principle of what they are calling IP compensation.
The district court did not completely agree to Apple's argument that 30% is fair (they expressly rejected it) and the appeals court observed that Epic's tactic on this argument was legally flawed and mentioned something to the effect of Epic should have used a different argument (and since they didn't, we are not ruling on this). To me, this looks like a vulnerability for Apple that they could be attacked by a different plaintiff using a different legal theory.