> But because of family reunification
There you go. You identified the actual cause yourself.
> the decision to admit one skilled worker actually means one skilled worker plus potentially dozens of other people with no qualifications whatsoever.
"Potentially no qualifications" is doing a lot of work there (strangely). What are the odds that only one person in the family is well-educated? And isn't IQ supposedly heritable? I'm personally neutral on family reunification. It undermines fairness but helps family cohesion. I understand arguments in favor of and against it, and there probably isn't a single right answer.
I'm not convinced family members without any qualifications or a means of supporting themselves in the US would even leave their country. What's the point? And if they're truly useless, their relative who would otherwise sponsor their immigration visa might prefer just sending them some money. It would be cheaper than financially supporting them here.
> instead they got Little Bangladeshes popping up all over the country
From the little research I did, Bangladeshi-Americans have higher educational attainment and household incomes than the median. America has always had ethnic neighborhoods - German/China/Korea/Japantowns, little Italys. What's the issue?