Americans already install alternative messengers on Apple and always have. That's not a real risk. I work in Europe and the people I know commonly switch between WhatsApp, iMessage, and Signal. I still use the Apple messenger by default because it is a noticeably superior product to the other two.

Similarly, my experience is that software produced in the EU commonly has worse support on Apple, including at companies I worked for. Again, that's already the reality. This fact was sometimes raised as a reason the apps had difficulty getting traction in iPhone heavy markets like the US.

There are reasons for Apple to stay in Europe but these aren't the reasons. I've commonly observed that the iPhone is more of a status symbol in Europe than in the US (where it is usually just the phone most people buy by default), which creates different market dynamics. In the US good interoperability with other iPhones is more important because that is a primary reason people buy them.