Apple does not require any consumer to use iMessage, nor do they make installing alternatives such as Whatsapp difficult. iMessage is simply a messaging option. This is in stark contrast to how MS treated IE back in the antitrust lawsuit days.
The fact that lots of people prefer to use iMessage -- despite myriad easily-accessible alternatives -- doesn't feel anticompetitive in the slightest; in fact making a product that people freely choose over similar alternatives is the definition of winning competition.
The Messages app is the only one everybody has no choice but to use though, since it’s the only one on iOS that does actual SMS, which is needed for interacting with businesses and in other scenarios. It’s also the most discoverable one and the only one that comes on the phone by default. It has a privileged place in the ecosystem, and that’s why it’s a potential target for antitrust regulation.
The fact that lots of people prefer to use iMessage -- despite myriad easily-accessible alternatives -- doesn't feel anticompetitive in the slightest; in fact making a product that people freely choose over similar alternatives is the definition of winning competition.