Islam at the gates
I have long wondered about this, too. As a fan of Western Civilization, I'll admit to being un-PC enough to feel some apprehension and regret about this prospect.
Sometimes I think that western culture's overwhelming advantage in producing economic and technological growth will carry the day. But other times I think that demographics are inevitably leading to a major decline in western values. A major question, I suppose, is how much the natives and immigrants are able to assimilate together into sharing a common culture.