Earlier quoted context omitted.
The best case outcome is to get publicity leading to US and EU antitrust regulators to file a lawsuit against Apple, both of which Apple loses. The conclusion of this lawsuit is that not only must Apple allow access to iMessage, they also must allow changing the default for every component of iOS - messaging app, browser, app store, let you replace Siri with other voice assistants - and to lower the 30% app store fee…
How is this an actionable anti-trust issue? iMessage doesn't even register as a messaging platform in the minds of most users globally. In the US is it dwarfed by at least three other platforms. Globally, do any of the other top ten (Apple is nowhere near the top ten) messaging apps allow third parties to spoof their service? The purpose of anti-trust is to increase competition and prevent unlawful monopolies. Apple…
But I do believe there's reason to consider Apple's policy relative to iMessage clients monopolistic. Apple's behavior is not significantly different from Microsoft's, which instigated US v. Microsoft [1]. That case largely took issue with Microsoft's mandatory bundling of IE with Windows and the extent to which Microsoft created an inorganic monopoly. In addition to how Microsoft's monopoly came to be one, the judge also took issue with Microsoft's methodology in quashing threats to that monopoly. One could claim that Apple is taking similar quashing action relative to Beeper now.
Microsoft of course appealed the judgement, and prevailed. But they prevailed only because the judge had broken his code of conduct in discussing the case with media; not because Microsoft's behavior was not monopolistic.
I don't believe global or domestic iMessenger usage is relevant.
[1] https://en.wikipedia.org/wiki/United_States_v._Microsoft_Cor....