Brexit happened. No one really wanted it to happen. Not those who set it in motion, not those who for months shouted about it with alacrity and, least of all, those who voted for it on a lark. But despite everyone, here it is, and it opens up a vast range of possible futures that are slightly less optimal than remaining.
One possible future is that some arcane part of the British body politics mobilizes and declares the whole thing null and void. It's unlikely, but it could happen.
Another possible future is that things tumble around for a bit, and then settles into a situation akin to that of Switzerland or Norway. While not formally part of the EU, they're not not parts of it either, and for most intents and purposes the difference is metaphysical.
A third possible future is that things break down completely, and the EU starts to treat the UK much like it treats Syria. No one is allowed to enter, and those who try are discouraged in the most direct of ways. Economic cooperation is severed completely, and the Brits are left to fend for themselves on their imperial isle. No matter how miserable they become.
The future that will actually take place is likely to fall somewhere in between. But it is worth noting that the EU contains the possibility of both possibilities, as it were. And any analysis of the EU as it actually exists - with and without the UK - needs to take both possibilities into account. The EU is both, in potentiality and actuality.
As it stands, brutal negotiation await those unfortunate government officials who have to hammer out the details. The EU has the capacity and incentive to not give two flying figs about the plight of the British people, in an effort to discourage further withdrawals from the union. As such, they will play the hardest balls right in the faces of the UK officials, who neither wanted nor prepared for the task foisted upon them by their supposed leaders.
It is not a pretty sight, and I do not envy those involved.
Especially not the Syrians.