I suppose it really depends on your values; some people value cultural cohesion and continuity of their historical traditions more than increasing the GDP growth rate. I guess I would support trying this out somewhere though, ideally an ultra-liberal country where racist wreckers won't sabotage the noble experiment. Maybe Sweden?
Liberal countries do want to provide a social safety net for everybody who resides in their territory. That means you can't simply allow all African's to migrate to the country without straining that safety net.
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