We should not support migration to white countries from anywhere, except white refugees from Zimbabwe and South Africa, including other whites moving from one Western country to another, except when engaging in the massively important task of seceding.
Because contemporary whites moving among white countries tend to be multiculturalists and others bringing harms. Even those less inclined toward globalism, hedonism, and multiculturalism are easily shamed with irrational guilt, thinking that if it is permissible for them, then billions of nonwhites should also have the right to move to the West. "I was an immigrant, too." Nevermind, the massive behavioral differences among immigrant groups.
Yes, white migrants helped make America a decent country, but today's white migrants bring more ideological baggage. Like nonwhites, they tend to view new lands as merely places to make money, having little attachment to the land and people.
There is also the risk of brain drain and a race to the bottom, with some white countries poaching white talent from other white countries, leaving the white country invested in developing that talent worse off.
There is also the rhetorical problem. When multiculturalists concoct fallacious economic stats and small sample fallacies touting the alleged benefits of migration, one of their tricks is to lump all migrants together, touting the benefits of nonwhite immigration by listing migrant white or Jewish inventors and entrepreneurs. I wish I had a dollar for every time I heard someone try to support third world invasions by listing the likes of Andrew Grove as a benefit of invasions.
Whites must view their homelands as a part of themselves that must be reformed. They must not view homelands as disposable wastelands, temporarily useful before escaping someplace else until the day cultural Marxism conquers the entire planet.
The big exception is secession. Ideally, hundreds of new white only countries would exist where individual whites could pick their citizenship, united in a military alliance, but without the horrors of cultural Marxism.