Like most approaches to antitrust enforcement, this one is probably meaningless at best, and might also result in the loss of choice for customers.
Apple will probably make it very, very challenging to access this "open" iMessage outside of EU, and/or will make it very uncomfortable to use (especially for competing developers, but we've already seen this with the blue/green imessage bubbles)
Microsoft responded to a similar 'integration' sort of scenario with neutered versions of Windows (such as Windows N/Windows KN) that almost no one outside of the targeted areas would choose, which is almost certainly similar to how Apple will respond.
They might also release different versions of hardware with different components missing. In fact, Apple has already responded this way already, with regional iPhones:
* Japan iPhones have an obnoxious shutter-click sound because that's required for all cameras, so people know when they're being photographed.
* Chinese iPhones have no option to make Audio Facetime calls, only video.
* United Arab Emirates (UAE) do not support Facetime at all.
So, I don't see that this will likely result in much change at this point. Apple has proven itself to be smart, wily, and very aggressive about flouting laws or regulations that might reduce its stranglehold on its customers.