The real danger is that if we slam the door shut on new immigrants, our culture will gradually become stagnant and parochial like the countries most of our ancestors fled. In those countries, the defining cultural attributes center around things like what kind of clothes you wear, what kind of food you eat, and what sports you play. We’ll know that American culture has truly ceased to be distinctive when we start to define ourselves primarily by our shared love of American football.
Read the whole thing for context. However, Derbyshire has an important point in his original post, though it wasn't what he intended:
You may think it would be good for them to have their nation so transformed, but they don't believe you. They like their culture. They're attached to it. They don't want to see it transformed in ways they do not approve, and have never voted for. This is called "conservatism."
A Hayekian view of markets and an authoritarian view of culture is indeed what makes conservatives thus. It's time to drop the collectivist schtick and pick up methodological individualism to analyze culture. Top-down imposition of culture is historically about as successful as top-down imposition of market structure: not at all. If our culture was largely determined by votes rather than free interaction among individuals, we would have just as much trouble as, well, the French do now with their economy.