Monopolies are not illegal - using a monopoly on app notarisation and "trusted" binary storage, to create a secondary monopoly in payment processing* is where most antitrust authorities will, I suspect, be concentrating their cases.
*These are separate business processes, with no natural link, as evidenced by their being separate for the entire history of online software purchasing prior to the appstore. Apple's attempts at arbitrary bundling those separate services together, does not fundamentally alter that, any more than Microsoft bundling Internet Explorer with Windows, and claiming it was "part of the operating system" worked for them when the antitrust authorities came calling.