Creating a perpetual outsider/underclass is hardly the direction I think we should go. I'd also point out that our culture has greatly been enriched, rather than degraded, by cultural infusion. It has made our lives, entertainments, foods, and languages, to sample but a few things, far more interesting than if we were all (well, actually, not many of us would be here at all, come to think of it) simply descended from the British puritans and cavaliers.