You don't understand, Rig.   The young underclass is people born in France (or the US, depending) who know no other life than life in France, and still are not citizens.

It is absurd to expect them to go to, say Algeria, if that is where the parents were born.   They have no connection to Algeria whatsoever.

There is no place to go "back" to.

They are rootless anywhere--and if they are poor, that is more tinder for the fire.