I don't think that's necessarily the right way to look at iMessage adoption, for some countries is was more about the cost and prevalence of SMS. In the US SMS was pushed rather heavily, but not that cheap, in other countries SMS was pretty expensive, and not really pushed that hard, so there was ample space for e.g. WhatsApp. Then there are countries like Denmark, where SMS was already either literally free or as good as free, before the iPhone and iMessage was ever a thing. In that case people already used SMS heavily and had no financial incentive to switch to anything else, so when iMessage rolls in people just adopt it because it's the default SMS app.
More than anything I believe it's more related to social circles than specific countries. I use Signal, but in frequent, because that's what colleague around the world use, and I have two friend on Signal, but that's it. It's not something I consider standard. I know people who use WhatsApp pretty extensively, but again only within a limited social circle. A lot of people use Facebook Messenger, but I've never seen anyone expect that I have it installed.
SMS is still the default, and sometimes it just happens to be delivered via iMessage.
More than anything I believe it's more related to social circles than specific countries. I use Signal, but in frequent, because that's what colleague around the world use, and I have two friend on Signal, but that's it. It's not something I consider standard. I know people who use WhatsApp pretty extensively, but again only within a limited social circle. A lot of people use Facebook Messenger, but I've never seen anyone expect that I have it installed. SMS is still the default, and sometimes it just happens to be delivered via iMessage.