The reason why diversity/immigration is bad is complicated. It rests upon a set of independent principles/data/findings which, when all put together, adds up to a utilitarian slant towards white homogeneity. So let's just go through the steps one by one, so that everyone can follow the reasoning until the inevitable conclusion emerges.
1) People's traits and tendencies at the macroscale are immutable. This is proven by hundreds of years of consistent IQ testing all across the world, by the Minnesota Trans-racial Adoption Study, by Rushton's Race, Evolution and Behavior, as well as every other study ever done by mankind. North Koreans, despite living in poverty, have not suddenly grown dumb. Nor have black children of rich black parents scored as well on the SAT as white children of poor white parents. When you zoom out and look at any race or ethnicity or nationality, the results always stay the same across any length of time, regardless of their circumstances.
2) People from every race, ethnicity or nationality can be well-behaved, nice, productive people. Let us even say the majority of every group is like that. Nevertheless, due to the fact that it is easier to tear down than to build up (look at the one single truck driver in Nice who killed 84 Frenchmen in a few seconds who had spent their whole lives building themselves up), if any group has even a small minority of destructive individuals, all of the benefits of the nice majority are wiped out, and you in fact end up in the negative. This is why El Salvador, Haiti, Syria, Somalia and the like are all shitholes, because the influence of the destructive minority far exceeds the influence of the productive majority.
3) It is impossible to distinguish the good from the bad ahead of time. If there were some magical test that could tell what was in people's hearts, there wouldn't be any crime or adultery or lies in the first place. Humans have evolved to deceive incredibly well, to the point that there's simply no way to separate the wheat from the tares. Oftentimes, 'wheat' from a foreign group spontaneously mutates into 'tares', which they themselves had no plan on doing until the moment it actually happens. If they can't even see the darkness in their own hearts waiting to pulse upwards and seize control of the sentience throne, then how the hell are a few bureaucrats going through checklists at immigration and customs sites going to manage it? The only data we can have on an individual is his statistical likelihood of being good or bad based on the group he belongs to. If you belong to a net-destructive group, odds are you're a net-destructive individual, and that's the best assessment we can possibly do as to your utility beforehand. Afterwards it's already too late, any assessment is pointless, because both the costs and the benefits have already accrued. So waiting and seeing is also obviously out of the question. Once America or Sweden or whatever has been reduced to El Salvador, Syria or Somalia, it's too late to say "Oops, I guess our screening process was wrong."
4) Not only must immigrants make up for all the duds in their midst if they wish to contribute to their new homeland, they must be so much better that their net benefit exceeds the natives just having another white child in the normal manner instead. The marginal gain of an immigrant must at least exceed the expected value of a normal Swede, German, Frenchman, white American, etc, to even start discussing why we should be letting immigrants from abroad into our country.
5) Many harmful traits of foreign cultures/nationalities are hard to even conceptualize or describe, and yet they obviously exist. For instance, Chinese and Vietnamese seem like smart, hard working, law abiding fellows, and yet somehow when you put them together, in aggregate, you get China and Vietnam. These countries are terrible, a huge step down from the countries white people make for themselves when left alone, like Switzerland and Australia. I may not even be able to pinpoint what I find objectionable about a Chinese immigrant, but the lived-experiment of mainland China speaks for itself. Why would we want another China overseas when we already saw what the first one did with itself? Why would we want to become more Chinese when we're already better than them?
6) There's value in diversity. Even if prospective immigrants really are superior to the natives, so long as they aren't strictly superior (ie, we have absolutely no advantage over them in any field), there's no reason to invade and repopulate a country with a new breed. Japanese, for instance, should not replace Europe and America, despite being better than Europe and America, because there are strengths white people have that Japanese lack. If nothing else, our looks and athleticism. But I also feel our rebelliousness, creativity, ingenuity, martial prowess and other traits clearly stack up favorably to Japanese. I prefer Japan over anywhere else on Earth, but I prefer a world that includes both Japan and Europe to a world of just pure Japan.
7) This logic extends outside the field of immigration. Point #6 is the same reason all children shouldn't just be made into women, despite women on average having higher utility than men. Point #2 is also the reason why we don't have to put up with gays, despite some gays being swell people. And so on. So long as a group is net-destructive, or at least net-not-more-productive than just a normal additional straight person being born, that group can be rightfully banished/shunned/eliminated for the health of the community.
8) Once you allow diversity into your country, you must treat it fairly, because people are repulsed and revolted at the idea of doing injustice to others in their midst. Imagine a girl who has fallen in love and wishes to miscegenate. No matter how many statistics you pull up showing her black partner is likely to abuse her, abandon her, go to jail, or give her an STD, she'll just say 'this one is different' and insist on following her heart. Tearing that couple apart by force would be a crime against the sanctity of love, which is just impermissible from a moral standpoint. And yet most of these lovers will quickly find out we were right and they were wrong, and suffer horribly due to their decision. Once that black fellow was in our midst, the entire train wreck was unavoidable. For the same reason, our revulsion against injustice means we can't have segregation, apartheid, second class citizenship, slavery, or any other setup that may ameliorate the bad consequences of diversity. Once they're in they're in, till death do us part.
9) For thousands of years, whites have shown that they are the sole sources of artistic, philosophical and technological breakthroughs in the world. We have a divine mission to continue this legacy in order to safeguard and advance mankind. We can't stay on this world forever. A nuclear war, asteroid, or the sun going red giant will inevitably happen. Therefore, as the stewards of all life in the universe, we must first and foremost preserve ourselves, so that the spark of creation, the future, and hope can live on. If anyone is going to develop the tools necessary to spread throughout the galaxy and the universe in a form that is both resplendent and sustainable, it obviously has to be us. Which means we can't be bogged down, or even pulled under, by all these third worlders with their third world concerns like crime, poverty, health care or whatever.
10) Even if it were possible with the perfect law code and the perfect set of behavior and the perfect institutions and so on to uplift under-performing groups, it isn't worth risking the cradle of civilization and progress, which means risking all life in the universe for the rest of time, on such a pointless and petty mission. Trillions of people will be living in the future, or not, based on the decisions we make right now. Mankind is at the crux of an immeasurably important moment. We can either extend to the stars or destroy ourselves, and it is immeasurably important that we get this see-saw teetering in the right direction. The idea that we should take a break and devote half our budget to helping out El Salvador and saving it from itself, even if this were possible, is patently absurd. Talk about missing the forest for the trees. If the world runs out of carbon and we all starve to death because plants can no longer grow, it won't matter whether we provided AIDS relief to the starving children in Africa beforehand. But if we're an intergalactic empire, we could solve such problems with the wave of our hands. Investment in the future, not frittering our time away on backwards cultures, is the only solution to anything and everything.