A review of a book that discusses this and raises this as an issue.
http://blogs.lse.ac.uk/brexit/2018/07/03/book-review-collapse-europe-after-the-european-union/I'm reading at the moment Flash Point by the eminent geostrategist George Friedman, who thinks it is not a matter of; if the EU will collapse, but when and Europe's history for the last 500 years has been one of many European nations at war with each other in its bloody history which has largely but not entirely been on pause since 1945.
Much will depend not only on the geopolitics of the member countries but whether NATO survives and if the US continues to offer Europe security guarantees (which might be withdrawn as quickly as this month by Trump to some European countries at July's NATO summit, where Trump has ordered the Pentagon to draw up plans to remove all 35,000 US forces from Germany and Poland has offered $$$bns for US bases there as a buffer against Russian expansionism). Then there is what role aggressive revisionist Russia plays, increasing global dominant aggressive Communist China, any US deterrence and how strong the European armed forces will be from the current largely pitiful levels, including the UK's when preserving European countries against each other and the other competing global powers. There is no doubt the 1945 rules based system which has provided the West's tremendous wealth and democracy is in decline and may well at short notice collapse.
History tells us that political and military vacuums encourages other expansionist powers to take advantage and fill the vacuum. The Middle East is currently an example of this with US withdrawal, Russian involvement and proxy wars between the competing regional powers of Iran, Saudi Arabia, Turkey and Israel being played out in wars in Syria, Yemen, Iraq and terrorism from Palestine in Israel, along with Hezbollah, ISIS and Al Qaeda.
If people, politicians and geostrategist are increasingly thinking of this as a likely outcome, so must we and our politicians as it will inevitably affect us and that might be at a minimum through falling European and global trade to a full blown war between competing Europe states against each other or Europe being the centre of competing global actors. There is also the issue of demographics, where we don't breed enough to replace ourselves and mass migration and their long term effects on European society is one of it results, where we need the work forces to provide Europe's pensions, health and social security systems.
With all of these competing factors what is certain is that over the next 20-30 years, which will include most of us on OOF, is that Europe is going to change considerably and probably not for the better. It certainly looks likes all Europeans are going to living in increasingly interesting times.