First to go will be the welfare states. Shrinking native populations cannot generate enough taxes to accommodate masses of immigrants with so few skills as to be effectively unemployable
It was not supposed to have happened this way. In 1995 a number of EU member states signed the Schengen Agreement, integrated into European Union law in 1999. The signatory powers promised to abandon their internal border protection in exchange for a promise by the EU authorities that they would police Europe's external borders. Then the EU authorities, while demanding that the Schengen states keep their borders open, spectacularly failed to honor their part of the agreement. There can be little doubt that the EU packed up, walked out and left its populations to their own devices.