Imagine there is an enclave populated entirely by foreigners, let's say Arabs. Chances are that this city will not be a net contributor of funds to society. How is this different from just incorporating an entire Arab city in say Syria in the social benefit system? Well, there isn't. And as we don't send money to the Arab city in Syria, why should we do it back here? Well, we shouldn't. So we should stop. But basing the welfare state on ethnicity in that way would be racist, so we'd just rather dismantle the entire welfare state instead.

Well, that's how the standard right-wing argument goes, certainly.

First question: why is the enclave not a net contributor to society?

Second question: why don't we send the money to Syria?

Third question: doesn't all this just depend on the story you tell yourself about who belongs in the in-group you believe is deserving of social welfare? Seems to me that right-wingers choose to tell themselves a story that restricts deserving to a small sub-group, at the extreme restricting it to small, lucky groups of the racially and/or religiously select.  The homogeneity argument is an excuse for your choice of story.