After all, the reason for high birthrates in their home country is for social and economic security but that won’t be needed in Europe.

Are you saying that after the educated folks with their systems of laws and dependable business models have been overwhelmed by the masses of uneducated high-breeders with their tribal systems, there will still be any form of ‘economic security’ lasting until their third generation settles down into low-breeding luxury? What sort of economy would support this model?