Epic is not suing on behalf of consumers. Epic's argument that Apple is the monopoly when it comes to service providers on the iOS platform. The counterargument that Epic could leave the iOS platform altogether because the consumer still has a choice to access Epic services on Android is like saying that the town residents could always move to another town. This is undue hardship that doesn't counter the anti-competitive behavior established by Apple on the only app store that they allow.Here's the catch: the desktop/laptop software distribution system does provide more than one store for purchasing digital goods, but the prices for consumers are generally either the same or higher than for mobile. The App Store doesn't actually have a reputation for software prices that are too high. If anything, the criticism is that it doesn't support high enough prices. If you go to web sites that are run by developers for macOS and look at the prices for software, you're not going to see very many $4.99 or lower apps. They're obviously not expecting millions of customers to find their site.
Let's change my illustration a little. Say the mayor of the town is also the owner of the store. The mayor refuses to allow any other stores to come to the town. The mayor (as the store owner) sets excessive margins. When the residents file a law suit, the mayor argues that the residents can always move to another town. Will this hold in court? You be the judge.