The opposite is true in the US. I'd guess that 95%+ of iOS users in the US text primarily on iMessage (excluding those who are primarily texting international contacts).
I'm fact, that's probably the biggest reason I think Apple has been able to abuse their iMessage stance for so long: the EU has been far stronger at regulating compatibility and interoperability (we can thank the EU for the fact that we can finally get rid of tons of dongles), but I think they've been less vigilant about iMessage solely because it's much less of a factor in Europe.
It would be pretty hard to argue that iMessage has a market domineering position in Europe and therefore not appropriate for the EU to regulate it.
That being said, I hope that once the other services are forced to interop apple will have no choice than to do the same.
It's probably hard to imagine in the US but people here absolutely will abandon iMessage over this. Their market position is weak enough that none of my family/friends/acquaintances who own an iPhone has even considered asking for anything but the "default" aka WhatsApp.
Nothing prevents Apple from offering a non interoperable format in addition to the interoperable one. For example, offer both MIMI and Apple iMessage protocols in the iMessage app.
And it sucks so horribly to the point that it's nearly unusable in modern group chats (or, rather, MMS is the protocol that's used for group chats and that one also sucks horribly).
