It probably won't be that bad at all. Norway gets along with the EU just fine. Regional governance is a double-edged sword, and it doesn't cut equally for all members. The U.K. and the EU will most likely make nice in time, and have perfectly reasonable treaties. If the EU is worried about losing members, then they have their goals and/or implementation to blame. It is their job to serve the members interests such that the EU is the more attractive choice. Of course, there is an ugly wave of xenophobia fueling this, but that's not all it is. The entire world is going to be increasingly confronted with philosophical questions regarding 'state', 'citizen', 'individual', and 'self-determination'. Political structures always change as technology does.