They have the war to lose. They know once that line is finally crossed there will never ever be a going back and the know They have 0 chance of winning said war. If they did kill enough white people they would end up with no white people society to live off of. Still a total loss for them. Even so there is basically zero chance the blacks could come close to accomplishing that task. The Muslims are better warfighters for sure but again can only claim one real victory over any white society and then were easily routed and dismissed when white people tired of them in Spain.