The point is unlike a real monopoly which is due to real entry barriers (e.g. train operators need train tracks, internet companies need cables, phone operators need antennas and satellites, etc.) the mobile OS isn't a monopoly or duopoly. Neither Apple or Android were ever the only mobile OS providers. There was Microsoft over a long period of time, Blackberry, Nokias own operating system and many other smaller ones. Neither Apple or Google have any advantage which Microsoft or Nokia didn't have either in consumer base and market share of mobile phones. Same for Blackberry. The only difference is that consumers have actively rejected the competition because Apple (and Google) exactly delivers what they want.

Now arguing that consumers are disadvantaged because they don't get what they want is falsifying the actual state of the market, when really they get exactly what they want and it's only some bad actors like addictive abusive gaming companies or other dodgy businesses which are doing more harm than help to our society and they want to force Apple or Google into opening more up to allow them even shadier practices.

Nothing stops anyone to create a more open mobile OS. There is no actual barrier to enter like in what real monopolies or duopolies have.

EDIT:

It's also important to remember that Apple hasn't invented their strict walled garden after Microsoft, Nokia and Blackberry left the competition. They always had their walled garden as a feature, and that is proof that customers actively chose to use Apple despite having a healthy competition of other open marketplaces, which clearly didn't deliver what consumers wanted. Consumers don't have the time to vet everything themselves. They value Apple's proposition and are even willing to pay more for an app on average than on any other mobile system. People change their phones every 1-2 years and if the walled garden wouldn't appeal to consumers then we'd see everyone have an Android by now for a very long time.

So you say: if Apple rejects your app, just ask users to reinstall another OS or buy another device?

Yes, I say if Apple's walled garden feature doesn't appeal to a user, then don't buy an iPhone.

If Apple was to change how their App Store operates and it stops appealing to the mass, then the mass will react and Apple will see sales drop over time and consumers will migrate to Android. Not the next day, but it would certainly happen like it did for Nokia users, Blackberry users, etc.

However, Apple didn't change their App Store guidelines. Users who bought a phone get exactly what they got on the day of purchase. It's Epic who tries to violate a feature which consumers have purchased and now Epic is suing Apple for having such a feature to begin with. This is not Apple vs. Consumers. This is a gaming company not finding a way to apply their shady practices on the Apple consumer based and they are pissed off. Consumers are happy for it though.

Isn't the barrier to entry that Epic needs to become a phone manufacturer? It's not like phone users can sideload another OS to play fortnite either.