IANAL and this isn't an argument of whats legally a monopoly in the US, but I think "dominant" or "not dominant" across the entire population is an insufficient

IANAL and this isn't an argument of whats legally a monopoly in the US, but I think "dominant" or "not dominant" across the entire population is an insufficient way to look at market power when network effects are involved. As an example, in the US its about 50/50 but Gen Z is 87/13 towards iPhone and there switching to Android presents a much much higher cost than a 50/50 or less scenario. Apple has considerable market power over a large slice of the population that network tightly together. You also see incidental enforcement of social stratification where wealthier users are more likely to use iPhone. I think those are things that someone could be legitimately concerned about from a competition standpoint without considering iMessage to be dominant across the entire market.