I can't speak for what other people have posted, but there's, to my mind, a quite analogous case for this situation: Kodak v. Image Technical Services.
The case involved Kodak refusing to sell parts for its copiers to outside repair shops, and they were sued by the shops who claimed this was anticompetitive behavior.
Kodak made a claim that since there was robust competition in the market for copiers, they couldn't have market power in the aftermarkets for "Kodak copier parts and service." The Supreme Court rejected that argument, holding it was possible to show that there was market power (and therefore antitrust liability) in a secondary market even if the primary market is competitive.
The key point in the Kodak case was that Kodak changed their policy on selling repair parts after people bought the original product, and the change in policy is what led the Supreme Court to decide that the market for repair parts was a separate and relevant aftermarket, because the people that bought Kodak copiers lacked information at the time of purchase about Kodak's repair policy and therefore were locked-in to buying repair services from Kodak after the fact.
Subsequent cases since Kodak have continually narrowed the scope in which Kodak is applied, and it likely would not apply here because people who buy iPhones have known since the App Store launched in 2008 that the App Store is the only place they'll be able to install apps from.
Yes, the specifics of the case don't match up perfectly, but the context of the question was "show me a case where a single company had antitrust liability without a majority of the market," not "prove with a single citation that Apple will lose."
The case involved Kodak refusing to sell parts for its copiers to outside repair shops, and they were sued by the shops who claimed this was anticompetitive behavior.
Kodak made a claim that since there was robust competition in the market for copiers, they couldn't have market power in the aftermarkets for "Kodak copier parts and service." The Supreme Court rejected that argument, holding it was possible to show that there was market power (and therefore antitrust liability) in a secondary market even if the primary market is competitive.