Well, it's not like they are going to be exterminated. I'm Polish, Finnish and Italian. My daughter is Chinese, Korean, Polish, Finnish, and Italian. I hardly consider her to be a loss for Western Europe or a gain for Asia. If anything, she is a loss for geopolitical boundaries and recessive genetic abnormalities.
They're not going to be exterminated, they're going to choose to let their genetic heritage, of whatever value, go. Along with cultural heritage, natch. I have no idea where you live, but in most of Europe the mixing of genetic heritages has to do with either trade or invasion routes. Or both. Today we mix our heritage differently. But if a society has lower than replacement growth for indigenous population, they have a certain period of time until they're ... gone. Math wins in the end. -XC