As I understand it it's not even purely race mixing that's a problem but race replacing. I wish every white nationalist would get their DNA sampled so they can see how pure they really are, even just among white people! But anyway broadly-white-peoples are breeding below replacement rates while broadly-(Africans|Mexicans|Arab|Turkish) ethnic groups are not. As a refugee/immigrant to Germany laid it out at some protest, Germany will become a majority-Muslim ethnically-Arab country within some number of decades just from sheer population growth of one group over the other, and what will remain will not be Germany even if it still calls itself such. A white nationalist then is just a person who doesn't want this to happen to their white country. Of course there are the more insane white nationalists who see Jews everywhere and so on, but nationalism is a spectrum you know? Japan for instance remains pretty xenophobic but they can maintain that without also being outright hostile towards Chinese, so the hostility towards Chinese has been going down.
If this was 1916 instead of 2016 I think the basic white nationalists would have a stronger case. This isn't a CURRENT_YEAR remark, but just a remark that timescales involving even 40+ years are ridiculously difficult to predict right now because technology overwhelms classic human nature, so when the impact of these things won't be felt for that far off in the classic human history case of technology remaining the same it's not very fruitful to dwell on it in the current reality of technology changing rapidly. That isn't to say they don't have any case at all but a lot of it is too little too late. They want to move to be more like Japan when they should really be satisfied with something more like Singapore. Not importing millions of foreigners permanently would also not be a bad idea but again, a bit too little too late.