Apple is for sure abusing their position. At the same time, the iMessage thing is a US cultural phenomenon as far as I'm aware. I've never seen anything like that outside of the US, because pretty much nobody is using SMS to communicate in other places in the world. Over the past 15 years I've been using various google chat platforms, Telegram, WhatsApp, Line, WeChat, and others. There is a pretty healthy competition in the messaging world. And creating new accounts to get features of a more interesting platform is a very low barrier of entry.