What opportunity cost? Diglossia is very, very common worldwide, and many languages have a different functional distribution. In Morocco, kids may speak Berber

What opportunity cost? Diglossia is very, very common worldwide, and many languages have a different functional distribution. In Morocco, kids may speak Berber at home, Arabic on the street, French in school. In other places, you may learn math and science in an international language and literature in a mother tongue. Bilingualism is very common worldwide; people being bilingual isn't actually a big problem that you need to use state resources (schools and education policy) to stamp out mother tongues. And as Joshua Fishman points out, if society isn't supporting minority mother tongues, then it is opposing them.