This seems like an unjustified assumption that Epic would attempt to leverage any monopoly they had on game engines to create a monopoly on payment processing. They haven't done this on desktop where they (up to legal issues) could, so I don't see why you think they would on IOS.

Moreover, there is competition between game engines and gamedevs are not a captive audience to quite the same extent as consumers are.

Moreover, if epic wins and they attempted this they would have literally established the precedent for Apple to sue them in return.