It's important to remember that Apple and Google only have a Duopoly, because nobody else seems to be able to develop anything that is remotely appealing to customers. Microsoft tried with their own mobile operating system many years ago and they failed at every attempt.

If Apple was told to open its App Store they could literally just shut down their App Store instead the next day and in a few days or weeks consumers all around the world would put too much pressure on any legislator to revert their changes, because not a single consumer cares about Epic, but everyone cares about their iPhone to remain as awesome as it is today, ad part of it is Apple's walled garden.