The same people claim the app store is this magically perfect service that provides infinite value for both developers and users. If that's the case no one would choose to use a 3rd party app store, so having the option would make no difference.
The reality IMO is that developers would try to flock to an app store with better terms and users would cling to the existing model.
To me, a fantastic outcome would be if Apple gets forced to allow developers to inform users they can pay for subscriptions outside the app store and to charge +30% to people that still want to use the app store. Then all the users defending Apple can put their money where their mouth is.
Then add in competing app stores that don't take a 30% cut and all of a sudden you'll have "dedicated" Apple users flocking to the new stores to save a buck.
The monopoly that Epic is complaining about is not a monopoly on smartphones or smartphone operating systems. It's a monopoly on selling iOS apps and processing payments for iOS apps and content.
You cannot allow a global duopoly of companies that managed to become the infrastructure for a large part of our economy to take a cut of all or most business that is done on top of that infrastructure.
Imagine we had allowed that to happen with telephone networks or roads. Whoever came first would have been able to suck the air out of the rest of the economy.