As it stands I find it hard to believe that less than 45 million people in the EU use the service monthly

I believe it. Iphone has pretty low market share in EU and most people use third party apps like whatsup. So even iphone users are forced to use those third party apps.

I think a demographic analysis would be more important. If imessage only has 20% share in total but 60% among teens, then that should be enough to be to be considered gatekeeper. Especially considering that those teens will eventually grow up and probably carry that market share into adulthood.