Market analysis under US antitrust law doesn't work that way. You don't just get to choose an arbitrarily narrow market such that a single competitor by definition owns 100% of it, otherwise plaintiffs would basically never lose in court. Market analysis is done by examining actual consumer behaviors, and in general, markets comprised of a single competitor are not permitted by the courts in antitrust cases (unless specific exceptions are met).
Apple owns 70% of the mobile device market in the US, and is the exclusive app distribution channel on the that platform.
Web Apps do not work right (especially when compared to the other major mobile platform) and Apple is making little effort to improve the viability of this potential channel, instead choosing to delete web push tokens every week or so amongst other sabotage of web applications, while these same web applications work just fine on the other platforms that support web apps.
