It doesn’t have a viable substitute for me. So… seems like they’re successfully exerting monopoly pressure on some people.

I use iMessage for conversations where everyone is on iPhones and Signal for mixed groups. iMessage isn’t the only reason I use Apple products, but it is a reason.

They don’t need to have 100% market share to be behaving monopolistically. And whether you feel the impact of the monopoly is also irrelevant.