Basically, oralism causes learning disability: it literally makes people stupider. And so, failing to teach sign language to a deaf kid is basically considered a form of child abuse.

I wonder how much that remains true with cochlear implants; I would expect that cochlear implants + oralism (lip reading, etc.) > sign language > oralism alone.

... though I'm not even sure of the last bit; from Wikipedia:

Research along those lines continued, however, and studies have helped validate the assertion that children benefit developmentally, educationally and socially from modern oralist teaching methodologies like the Auditory-Oral method.Geers and Moog (1989) found that of a test sample of 100 profoundly hearing-impaired 16- and 17-year olds enrolled in oral and mainstream programs, 88% were proficient and highly intelligible with their spoken language, and could read at much higher grade levels than the national average for deaf children.

Do you have any sources for Oralism being worse than sign language, and not merely less popular among the deaf? (the latter is evidence, but weaker than serious research)