In this case, once you buy an iPhone, you're forced to use Apple's system to make in-app purchases. The tying product is the phone, and the tied product is the in-app purchase.
So you need to show that Apple has sufficient market power in the smartphone market, and that is where the question of the ability to control prices of phones comes in.
(Neither does Apple v. Pepper.)
But since you mentioned it, I'm not really sure that Epic will succeed with their argument that the "iOS App Distribution" market is the relevant tying product market absent an analysis of the overall smartphone market.
The government playbook in this sort of situation is normally to start subpoena'ing emails, execs etc. and look for hard evidence of actual anti-competitive practices. They're not only interested in a business model as it's described in the media, they want to know what company execs actually did. If they find something they file a suit.
Well, assuming it is one based on abuse of market power and not the other kinds of antitrust violations (e.g., combination in restraint of trade, etc.), it would fail if the plaintiff couldn't establish that Apple had market power in the market for the product for which they allegedly abused market power. If that was iPhones, for instance, that would be the market for iPhones plus whatever products empirically people substitute for iPhones in response to pricing changes. (Of course, if Apple has market power, there is some range in which the “plus...” part is “none”, since market or pricing power is the ability to raise prices without driving sales to a competing good.)
If the lawsuit was about selling some popular class of app in which Apple leveraged its monopoly over app distribution, I think the plaintiff could have a pretty strong case.
Keep in mind that even if Apple only had 1% market share in the smartphone market they would still have a "monopoly" over app distribution on their own phones.
How exactly does Apple "coerce people into paying more for their phones via a mechanism other than actually offering a better product or service", and what is that mechanism?
> But if a firm is able to drive up prices with impunity for any reason, it's likely they possess market power.
How is Apple actually able to "drive up prices with impunity for any reason"? It seems to me, the more Apple raises iPhone prices, the more people are priced out and decide to switch to a cheaper phone instead.