To use your analogy in this case McDonald's would be the only burger chain in town and if you don't like it then go to Pizza Hut. Doesn't really make a ton of logical sense. Consumers also generally don't leave iOS thanks to ecosystem lock in. The only market you don't see this behavior is in China thanks to WeChat which effectively acts as a second App Store that works on iOS and Android. Also while Apple may only have 20% global share they have closer to 50% in the US which the courts are far more likely to consider. Also "monopolies aren't illegal"? What on earth are you talking about? Anticompetitive behavior has been illegal in the US for well over a century, contrary to what recent administrations' actions may make you think.
I am aware of Epic and Apple's arguments. I don't think a relatively young and small firm that specializes in business law necessarily has the same grasp of antitrust law as the century year old firm Epic hired that has a long history in antitrust lawsuits and successfully best Apple in their patent lawsuit against Qualcomm.
I was using the analogy of McDonalds and Wal-Mart to highlight how stupid the argument is that Apple has a monopoly on Apples users, not that the actual businesses were directly comparable. Is consumers not leaving iOS because of "ecosystem lock in" really a well studied phenomenon with iOS customers or is it simply an argument Epic brings forth in their suit? Logistically it seems like the longer you're in an ecosystem, the more invested you're going to be in it, but that happens with... literally every ecosystem and if a company REALLY does something you don't like you would leave that ecosystem, investment be damned (most likely evidenced by the mass Xbox One exodus at the start of the gen)
Yes anti competitive behavior is illegal, but not all monopolies are illegal.
"A monopoly is when a company has exclusive control over a good or service in a particular market. Not all monopolies are illegal.
For example, businesses might legally corner their market if they produce a superior product or are well managed. Antitrust law doesn't penalize successful companies just for being successful. Competitors may be at a legitimate disadvantage if their product or service is inferior to the monopolist's.
But monopolies are illegal if they are established or maintained through improper conduct, such as exclusionary or predatory acts. This is known as anticompetitive monopolization."