There are more important things than the economic argument (which there have been very conflicting reports on across Europe) for mass immigration, for example that multi-culturalism has failed in the old European nation states (officially aswell according to Merkel, Cameron and Sarkozy etc.), and that crime, radicalization, uneployment, low education and lack of integration is rampant among especially Sunni Muslim African and Arab immigrants, even in the third generation.
Personally, as I care about culture and genetics/biodiversity, I think there's even greater issues in play.