I would recommend looking up what antitrust does and does not cover. This is not suitable grounds for antitrust arguments as the law is currently given; if you were arguing 'Apple has a monopoly on Apple-exclusive apps', you might have a case, but even then there are many apps that are cross platform, and those that choose to develop exclusively for Apple are making a market decision that they may be liable for if the cost of production increases because of it.
I don't think you can argue platform monopoly here any more than you could argue, say, that Amazon has a monopoly on AWS-specific services. Since an app developer can choose which platform to use with approximate levels of parity between those platforms and a few distinctive services per platform, it is my opinion that the 'platform monopoly' argument is rendered void. This argument is only weakened in recent years as feature parity increases between Android and Apple ecosystems.
Vertically integrating and excluding competition is exactly what antitrust covers! The monopoly that Apple has on the app store for the devices it sells allows it to exploit consumers with pricing. That's the case that the Supreme Court has allowed to move forward.
AWS is also a platform but they don't lock customers into only using AWS or AWS-licensed services. This is almost as preposterous as saying, "Someone bought it, so it's not anti-competitive".
All the vertical integration Apple has been able to muster hasn't prevented more open competition from growing in the smartphone market, so your point continues to remain moot.
Again, to make your case, you must argue that Apple products are categorically different from any other device available on market and that they are preventing new entrants from competing with it, which is so blatantly not the case here.
Addendum after edit time passed: the final and primary criticism I'm making about this argument is this: if you selectively narrow your view to the Apple ecosystem and ignore all external entities, you would easily come to the conclusions you are coming to. I believe that is an artificial and distorting narrowing of the scope that must be considered in this argument.
I don't think you can argue platform monopoly here any more than you could argue, say, that Amazon has a monopoly on AWS-specific services. Since an app developer can choose which platform to use with approximate levels of parity between those platforms and a few distinctive services per platform, it is my opinion that the 'platform monopoly' argument is rendered void. This argument is only weakened in recent years as feature parity increases between Android and Apple ecosystems.