A monoculture is almost always worse. The mix is what has made--and often continues--to make countries stronger. We can disagree about the quantity or blends, yet it seems obvious to me from insular cultures that we have to have more than zero.
Sure. I don't think even the most rabid anti immigration advocate would hold the number at zero, but using the GP's logic ("the mix is always better") the Native Americans should be quite grateful they were culturally enriched by the incoming Europeans.
