America without blacks and hispanics looks better on paper than America with them, and it potentially changes cultural norms and values, which many find scary. I think most of the poitive effects of having different cultures are harder to measure, particularly short term, as it's a lot harder to quantify new music, food, businesses, etc compared to violent crime. That said, anybody who is worried about a large scal takeover is a fucking moron who didn't notice the other dozen or so times there was a mass diaspora to the US and how we haven't been 'overrun' by Italians or the Irish.