[summerlight]

There are two factors that matters to a relevant market definition in monopoly; product and geographic. We don't know what product definition will eventually be chosen by the Justice Department yet, but Apple will be in surely trouble when it's defined as users with Apple mobile devices since Apple has an absolute power in this hypothetical market.

It's true that Apple doesn't have a dominant position in the phone market, but the same logic may not be applicable to the app store. Do not confuse those two; Not much customers change their expensive phone due to $1 increase of app prices which could be easily mandated by Apple even though there's nothing stop them from changing their phone. So Android apps are not really viable substitutes unless Apple allows another competing OS or app store on iPhone. Because of this reason, even Apple hasn't attempted to defend itself with "Android is a viable option!".

[mrkstu]

Which would make no sense, since it would be a circular argument that would apply to every company with a product to sell.

[fooey]

It's the argument that the car makers lost when it was determined you're not restricted to only their mechanics when it comes time for a warranty claim.

Ford can't restrict you to only Ford parts and Ford service mechanics and Ford Gas and Ford toll roads just because you bought a Ford.

[summerlight]

I agree that it's not very intuitive. That's why there's no single clear definition of "market" in antitrust laws and most of the time it's done by hypothetical monopolist test, which is a highly data-driven process. I have no idea on what will be the final decision since it's not a predictable one for the outsider; I am just giving you one real possibility rather than ruling it out from the beginning.

[anoncake]

How so? Most goods can trivially be substituted for another of the same category. A dishwasher can be substituted for another dishwasher etc.