No sane person says that because native English-speakers are not learning the fundamentals of writing, they should no longer be taught English. Or that history classes should be scrapped, because too many students don't know the difference between the Civil War and the Revolutionary War. The same way that schools shouldn't stop teaching history, they shouldn't stop bilingual ed. And the same way that instruction in history needs to be improved, bilingual instruction needs to be improved. Teaching history is a good thing.
Bilingual classes are a good thing.
They just need to be done right. The primary goal of bilingual education should be, of course, to teach English to immigrant children. But it should not stop there. There also needs to be an emphasis on maintaining a student's original language. Making a quick transition to English the sole goal of bilingual education is a waste of a student's native tongue for students who come to school already knowing French or Spanish or whatever other language.