Actually, I agree with you that the "Kingdom" is fragile, but then so is the European "Union".
The Problem is the Difference in Scale of Targets.
For the UK to Fall Apart. All the EU has to do. Is to Stay there and Wait while UK Economy Collapses.
As long as UK does not get any Profit from Leaving the EU. The EU will not only not Destabilize. But will even go out of this Stabilized because the Leave Parties lose alot of Influence.
This can already be Viewed Right now. Because while some Parties instantly upon Brexit Votum started Demanding an Referendum. The Polls Show that the Leave Parties are losing Influence because UK is Falling Apart and People are now Scared that if they Leave the EU the Same will happen to them.
For the EU however to fall Apart. The UK needs not only Calm down its current Problems and Independence Movements. It needs to Cut out a Deal much Superior to what it had in the EU. It somehow needs to Magically Increase its GDP and GDP Per Capita.
It needs to Fullfill the Promises as well as Avoid an Humiliation by admitting to EU Regulations and Gain Access to the Free Trade Zone for Free.
And it needs to do all of that very very Fast before the UK Breaks Apart.
The Challenge is Incredible Lopsided.
The EU to Win just needs to ride the Lift to the 10th Stage and Sit down there to Sip some Coffee.
Meanwhile the UK to Win has to Climb Mount Everest without any Climbing Equipment while Carrying a Horse on its Shoulders. And it needs to be on the Top and Finish an Surgical Operation on the Horses Brain with nothing but a Tea Spoon before the EU has Emptied its Coffee.
Its fairly Obvious how this is going to End.
The EU will end up being Stabilized more than ever because Everyone Sees what Leaving the EU Results in.
And UK will be very very Lucky if it manages to even Remain UK and does not go Back towards being several States of which some wont Apply for Immediate Return to the EU.