> iMessage isn't a monopoly on messaging nor SMS
People tend to misinterpret what it means to be a monopoly for antitrust law. The obvious example here is United States v. Microsoft Corp., where the inability to remove Internet Explorer was at issue. Being unable to set an alternative messaging app with equivalent functionality (such as Signal) on a mobile phone is at least as cumbersome to the user as being unable to uninstall Internet Explorer, despite the ability to install competing browsers. The iMessage case is arguably worse because the competing browsers had the same features as IE. Apple uses its control of the handset to disable other messengers from supporting SMS on iOS.
Also relevant in that case is that Microsoft was argued to be a monopoly not over all computers or even all desktop computers, but over intel-based personal computers. This illustrates that the scope of the market in these cases is often smaller than one might initially think.
The FTC has a page here that goes into it [0].
> Then courts ask if that leading position was gained or maintained through improper conduct—that is, something other than merely having a better product, superior management or historic accident....Courts do not require a literal monopoly before applying rules for single firm conduct; that term is used as shorthand for a firm with significant and durable market power — that is, the long term ability to raise price or exclude competitors.
In this case, forcing users to use iMessage for SMS decreases the likelihood that users will install an alternative messenger with a smaller feature set due to Apple's control over the OS.
That reduced likelihood compounds quickly because the popularity of a messenger tends to scale with the square of the install base.
[0] https://www.ftc.gov/advice-guidance/competition-guidance/gui...