What a fucking disaster. Britain runs away from hard work, like a petulant child. The EU now needs to stop anyone else from considering leaving, so their rulings against Britain will be draconian and punitive. Britain fades into an irrelevant island full of National Front fuckwits. The EU gets stronger in the face of this adversity, and redoubles the efforts of member nations to feel empowered over their own governance, to stave off the key reasons for Britain's exit. The financial hub moves away from London - which makes everyone happy because London is ridiculously expensive - to Brussels, Bern, Berlin, Stockholm, or any other modern city with history and room to grow. The 40% of Britain's value that is stored in property in the Southwest (aka London), tanks, and with it goes Britain's economy and buying power on the international market. Nationalist right-wing xenophobes everywhere see Brexit as a validation their causes, and every country in Europe (and the USA) see a dramatic spike in hate crimes. Brexit also validates Trump's presidential run, just as it was losing steam and about to fold in on itself, and the election is won by Hillary with a tiny margin, thereby legitimizing all the hatred and moronic policies of the Trump crowd... And... fuck. Fifty years of social progress gets thrown down the drain, and the world economic power shifts entirely to Asia.
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.