The relevant market isn’t the specific Apple device users, but all users and all their competing devices. This is typically how US antitrust law works as it is based on consumer harm. And that’s broad, not the specific consumers who choose to use a particular tool or be in a particular system.
https://law.justia.com/cases/federal/appellate-courts/F3/253...
> The District Court determined that Microsoft had maintained a monopoly in the market for Intel compatible PC operating systems in violation
Change "Intel compatible" to "A series compatible" and it's Apple. Change it to "ARM compatible" and it's still Apple given they have 60% market share in the USA. Only countries enforce anti-trust, there is no world government to look at market share it the world.
So people are so enamored with apple devices that they cannot stop using them, but don't want to pay the rent the company charges them... just go play somewhere else. Or create your own park. That's what Hawuei did, that's what Android did.
Consumer are locked into ecosystems so you cannot just say "they can switch to a competing device". Can they move all apps and content from the old device to the new device at no cost? Do they need significant time to setup.