multiculturalism is a liberal value. historically both the left and fascist movements have opposed it; the fascists because they feared the dilution of their Aryan blood and the contamination of Western culture, the left because multiculturalism always gives way to bourgeois nationalism. and that’s precisely why liberals like it.
multiculturalism performs two important functions in a liberal society. the first is that it expands the pool of cheap labor that corporations have access to - when you have settled communities of a particular nationality in your multi-ethnic state, it’s much more viable for foreign workers (who are already very poorly treated and badly paid in their home country) to immigrate. they’ll have a cultural environment they understand and are comfortable in; they may not even need to learn a new language. as laborers, they’re liable to accept working conditions that citizens wouldn’t, because even minimum-wage 9-5 jobs are a massive improvement over what they had before. this is of course a recipe for inequality along racial lines, which brings us neatly to the second function of multiculturalism.
where you have ethnic inequality, you have ethnic tension. even if the disparities between workers of two ethnicities are miniscule in comparison to the disparity with the bourgeois (which they always are), that ethnic disparity will be much more immediate, simply because the bourgeois are a small and insulated community that proletarians almost never interact with or see on the streets. (this is the “aristocracy of labor” in action.) W. E. B. du Bois, writing on race relations between black and white laborers, articulated a theory that has since been resoundingly vindicated: that the disparity between black and white workers was maintained by the bourgeoisie not because they hated blacks, though they may well have — rather, it was because as long as that inequality existed, they could rely on white and black labor being at each other’s throats; it was an effective guarantee that the two forces would always neutralize each other rather than join together in solidarity against the true oppressors — the bourgeoisie. this tactic effectively neutered class consciousness in america, to the point where otherwise solidly marxist parties like the Black Panthers, which did an enormous amount of good for black americans, rejected interracial solidarity until it was far too late. today, we don’t even have that.
multiculturalism is thus absolutely counter to the interests of the proletariat. in america, it is poison to the white proletariat, whose wages are depressed and who are deluded into blaming the wrong people, and it is poison to the nonwhite proletariat, the vast majority of whom are permanently trapped in poverty and ruthlessly exploited by the (itself increasingly diverse) ruling class. and this doesn’t even begin to approach with the whole other problem of culture clash — despite what liberals would have you believe, not every culture in the world is actually compatible with every other culture. (there’s a good analogy to be made to the abomination that the disability rights movement has become.)
this should give you an idea of why the koch brothers were so big on the idea of open borders — and why bernie sanders famously (and correctly) dismissed the idea as right-wing nonsense.
so what are the alternatives? do we just give up on the project of proletarian internationalism and resign ourselves to xenophobic, isolationist ethnostates? well, we certainly don’t have to. we have two basic options. the first one is the obvious one, the true “melting pot”: accept immigration but force people to assimilate, encourage intermarriage, and produce a new, unified culture out of the cultures that came before, ideally taking the best values and practices of both and dropping the ones that caused friction and conflict. i’m not aware of any successful examples of this; it’s kind of a pie-in-the-sky dream. it also means the elimination of linguistic diversity which… i can’t really argue against on practical grounds, but coming from a social science background — linguistics specifically — i hate the idea. and even if it worked, there’s no guarantee that the process of cultural intermixing would even peaceful; it could produce years to decades of horrible, destructive ethnic strife and leave a lasting legacy of pain and distrust.
i’m not really a diversity-for-diversity’s sake kind of person, but in practical terms, the second option, the more diverse option, is probably the better one, if only because we know it can work, because we have a historical example to refer to: the Soviet Union.
americans, on the right and left alike, tend to conflate the ussr with russia and soviet citizens with russians. while it’s true that the soviet union had its roots in russia and the october revolution, it’s important to understand that the ussr was, as its name plainly states, a union of soviet socialist republics. each republic was itself a nation-state (as opposed to the US, which is made up of just plain old states), and each nation had the right to self-govern, but also to participate in the governance of the whole. crucially, their right to self-govern did not depend on the size of their population. this would be akin to the US setting aside a whole state for, say, hispanic citizens; a state that they were permitted to control and develop in a way that best suited their common culture and needs, without disrupting any other culture, and preserving an entitlement to, say, welfare contributions from larger or more resource-rich states. there would be no room for ethnic resentment to breed, and nobody would be forcibly exposed to cultural practices or values that run counter to their own. the benefits of such a system should be obvious to europeans, because the EU has historically been quite similar.
and once you’re there, if you’re really dead set on that mass immigration thing, you can go hog wild, because the factors that make it problematic mostly disappear. but that’s only if you can keep income disparity between member nations under control, which likely will require extensive welfare systems — in other words, keep the system socialist.
but if you try it in a capitalist society? all you’ll succeed in doing is breaking the back of proletarians worldwide.