1.6 million illegal immigrants arrived in Italy alone the past few years.
If Europe let this go unopposed, the illegal immigrants will settle and gain political power and potentially even take over portions or eventually whole countries.
Exactly the same way the Western Roman Empire disintegrated, they allowed in hundreds of thousands of Germanic refugees fleeing Atilla. In a hundred years they were in control of Europe and North Africa.
They did try and fight, but loss of territory meant loss of income and taxes which affected their military and available recruits. Without the money for a sizable well-armed military to enforce their government, they were finished. Parts of Europe could go the same way.