The argument that there is a net social benefit to having a single culture.

Not at all. The argument is that Polish deserve this for having previously occupied the country. Or other nationalistic crap.

edit: just because you are accustomed to rationalizing actions as net benefit does not mean that other people do that. Most don't. Utilitarian arguments are quite rare. The proponents of said policies themselves never even claim them to be beneficial to anyone. They use other rhetorical tools.