VDARE editor Peter Brimelow speaks about the problem of immigration: it replaces the original population. Where America succeeded when it was English, German, and other Western European descended people, it will not be the same under a new group. In fact, it will resemble every other mixed-race third-world country, like Brazil or Mexico.
This is how all societies go out. Notice the vast amount of mixing in Southern Italy and much of Greece. Without the original population, two things happen. First, as Brimelow notes, the new population lacks the abilities of the original. It is chosen not for its self-sufficiency but for its willingness to do repetitive tasks. Second, and more importantly to my mind, the civilization loses a sense of identity and becomes more like a shopping mall than a culture.