I have to say that in Denmark at least, iMessage seems to thrive quite well. There are quite a lot using Facebook messenger, but SMS and iMessage is a close second. This is entirely from my point of view. Never met anyone using the examples you mention, unless they are communicating with foreigners on a daily basis.
Well… for now at least. Who’s to say that won’t be the next thing on their list. They do make a pretty penny on the play store as is, and could improve that if they banned side-loading. And let’s be honest, side-loading is probably a niche thing still.