I struggle with their motivation.

They may have anticipated that importing the multitudes with no discernable skills or talents would just serve to turn everything into a mess that would require ultimate control, something they crave.

If they're that evil and smart, then they would have also had to realize that they would be controlling a shrinking GDP and a rapidly deteriorating society. So they have ultimate control but it's not of France, it's of a 3rd world version of France. The very things that made France a first world power would no longer be present and it would sink to the level of the average resident.

I tend to believe they really thought they could have it all.