If this was going to happen, it would have happened during the peak of "Grexit" talk about Greece. Greece were doing incredibly badly out of the EU at that point, far worse than the UK could ever be said to be experiencing, and decided not to leave. That's why I don't think further fragmentation is likely, and the banking system has been shored up.

French exit would kill the EU, but I think we'll see another narrow defeat for Le Pen. But I could be wrong there, it's volatile.