I feel the mistake made by open borders advocates is completely taking social capital for granted and only seeing the results of economic growth ASSUMING the social capital was still in place (not shocking since they are economists I suppose). You can't have growth if there's no foundation to build off of. Open borders is to me an utterly shortsighted policy that would start out as encouraging an obvious plutocracy as workers economic and cultural power is totally broken before utterly spinning out of control in the form of various conflicts.
-The United States would likely become a de facto one party state as immigrants would simply vote for the pro-immigration bloc and the other party would represent natives whom view the entire government as a fraud. Bye functional American democracy.
-As civic nationalism is utterly destroyed by the shock effect of massive numbers of unassimilated immigrants entering the nation ethno-nationalism would become the primary defining characteristic in a way some liberals seemingly can not conceive of happening in real life. Ethnic gangs would spiral out of control especially as there is not large enough law enforcement agencies in place to contain them. Expect redistributions of the "new wealth" in the nation to be even harder to redistribute than it is now due to the utter lack of social trust between workers in different culture. Bye cultural cohesion, hi new era of crime and ethnic-conflict that would make the past 50 years look like a golden age in comparison (Fun fact: I have never seen a real response to this beyond calling the person a racist).
-Native workers would feel betrayed by their government, mainly because they literally were (as their government was supposed to represent them as opposed to literally the entire world). Expect the US government to be viewed as illegitimate to the native working class and right-wing paramilitaries to fill that vacuum where ever they get the chance. The US government could counter them though I imagine it will be tough doing so while also having to keep up with the massive infrastructure that is already not in place for open borders and which would already be lowering life expectancy in the nation. Bye relatively non-violent norms.
-Many of the immigrant groups themselves have no special attachment to liberalism. While the liberal party that may support this obstensibly would that won't help when it comes to referendums, culture and local elections. Expect reactionary social policies to make a come back, fueled by the fact the native far-right would now be massively powerful. Bye liberal social progress (unless enacted in an increasingly undemocratic top-down manner).
-After the last piece of social capital is annihilated I can't see how even immigrants are going to enjoy the benefits of being in the United States: high corruption, economic concentration of wealth, total distrust of neighbors, ravaged infrastructure and high rates of violence will not be a recipe for attracting capital which will simply move to other more stable states. Investors will prefer stability to risk. The things that build our human capital, cultural norms and education, would both be nearly destroyed for different reasons. Existing capital will be at risk of stagnating or deteriorating and needing to be increasingly shared between a now far larger workforce left to fight over scarce resources. The government would essentially be an overstretched empire trying to contain too many conflicts and groups at once and probably wouldn't even have total control over every part of the nation.
What I can't understand is why open borders, an insanely risky policy that even in the best case scenario its advocates acknowledge would damage the local working class, is favored over aid to third world nations in the form of copyright and patent reforms. The 'protection' on such things strongly favors the extremely wealthy and would allow third-world nations to develop more cheaply but no, let's go with the policy that might actually make the United States into a third-world civil war zone. Frankly, the attention on open borders shows some serious flaws in liberalism based on economic utilitarianism. Hand waving and saying "it will work out" is, well, no less silly than any other utopian ideology with an extremely narrow focus.