It is hard for me to imagine any browser makers switching to a non-Webkit-based implementation if they can only do it in the EU. They would need to support the Webkit-based code as well for other regions, which seems like a lot of work.
You are not wrong and that's probably what Apple is counting on. Especially since if you rely on services from outside the EU then those people can't really test their website on the non-webkit browser engines. Of course if more countries force Apple to allow 3rd party browser engines the scales could flip.
With the third-party appstore also being available in the EU, someone will surely roll a non-webkit browser, though I have no idea how hard that is going to be as I don't develop for iOS.
Am I wrong?