There are many reasons why the big cheeses among th eEuromaniacs are petrified of a "Leave" vote. One of them undoubtedly is that a Brexit will hasten the collapse of the whole house of cards.
As Svejk has correctly pointed out, the entire farce is creaking at the seams already and will eventually topple sooner or later. A Brexit will make it sooner. A vote to Remain will make it later. Either way its collapse will be messy (just think of the Global Apocalypse that is being forecast should just one country leave then multiply it by 28 plus VAT). When you are in a building that's about to collapse you get out. You don't stay inside and try to shore it up.