[Epistemic Effort: I thought of this argument and was so pleased by my own cleverness that I decided to post it.]
A lot of people are worried about immigration because they’re worried that immigration will dilute their culture: instead of being a place full of People Like Them, it is a place full of funny people with funny food and alien values. I actually do think this is a legitimate cost to immigration, but I don’t think that all immigration has this quality.
Every US state has open borders with every other US state. It’s true that the US state example isn’t precisely the same as fifty countries which happen to have open borders with each other, but all the differences function to make US states more similar to each other: for instance, we have a shared federal government that exercises a significant amount of control over our lives. And yet immigration has not served to make Alabama the same as New York.
In fact, immigration has probably made Alabama more different from New York! Queer liberals from all around the United States tend to move to New York City; I assume that conservative Christians from all around the United States tend to move to Alabama. Liberals are so stubborn about moving to big cities in blue states that it was a pretty major factor in this election: if liberals all stayed where we were born, Hillary might have won. I myself come from Florida (purple state), my husband comes from Wisconsin (purple state), and we both currently live in California (blue).
It’s pretty obvious why this is the case. I have absolutely no interest in living in a small town in Alabama. People might look at me funny for not going to church, I might get harassed in the bathroom, and it’s impossible to get socialist vegan pizza. Conversely, a lot of Alabamians don’t want to move to the wretched hive of degeneracy and decadence that is San Francisco. Given that basically no Californians want to move to Alabama, the only thing that closed borders between Alabama and California would do is keep vegans born in Alabama from fleeing, and therefore increase Alabama’s chance of having to put up with a socialist vegan pizza place.
You can recruit people to your culture in two ways: by socializing children born into your culture (vertical transmission) or by recruiting adults who have an affinity to your culture (horizontal transmission). Immigration has little effect on successfully socialized children, who are presumably going to stay part of your culture. But without immigration you’d have to put up with the unsuccessfully socialized children and you can’t engage in horizontal transmission at all.
Of course, this argument doesn’t work for all immigration. As far as I’m aware, there’s only one country that the US has de facto open borders with: Cuba. As the Miami Herald said in its Castro obituary, the US’s de facto open border with Cuba “transform[ed] [South Florida] from the southernmost tip of the United States to the northernmost point of Latin America.” Today, more than three times as many Miami residents speak Spanish at home than English. This is a pretty major cultural shift!
I think what’s going on here is that Cubans are not immigrating to the US because they feel like the US is a better cultural fit for them than Cuba is; they are immigrating to the US because Cuba is a horrible country. While early Cuban refugees might not have been enthusiastic about being surrounded by gringos who can’t speak Spanish, it was definitely a better option than being executed for being a member of the opposition. They are immigrating to a place where they had poor cultural fit because their other options were worse. (And transforming it to a place where they have good cultural fit, natch.)
Of course, this argument does not do a lot for pro-immigration advocates; most of us tend to care most about immigration from horrible countries, because the benefit of immigrating from a horrible country to a non-horrible country is much larger than the benefit of immigrating from a non-horrible country to a non-horrible country with better cultural fit.
However, the economic benefits of immigration still apply to non-horrible-country/non-horrible-country immigration. Increased immigration between non-horrible countries is likely to increase the cultural diversity between those respective countries and the cultural similarity within them. And since we’re only talking about immigration from non-horrible countries, we don’t have to worry as much about assimilation; the immigrants will already have non-kleptocratic liberal democratic norms.
Therefore, I propose that people who are against immigration should advocate for a policy of open borders for all citizens of Anglosphere countries. (Since the US is one of the more conservative Anglosphere countries, this has a further benefit for the average anti-immigration American Republican; the liberals would finally make good on their threat to move to Canada.) If all goes well, we can expand to include other developed countries, such as Japan and Germany.