I have seen many instances where people close to the deaf person discourages their use of sign.
For example, my high school ASL teacher used to tutor a deaf boy, whose mother was hearing (He was about 12 years old). He had a cochlear implant, which did nothing because not only was he profoundly deaf, but he received it long after the mother entered him in oralism programs. She thought that he was simply struggling with school and needed a tutor, when in reality, he could not understand what was being said because he had no comprehension of speech, and his mother refused to let him learn ASL. If my teacher merely spoke to him, he couldn't understand her. But if she also used sign (she experimented to see what he knew), he picked it up and understood. He was a whiz in math, but had a harder time with the other subjects because they all required reading. Since his mother was against the use of ASL (and my teacher could not go against what the mother wanted), he became extremely frustrated with the situation and no longer wanted to learn. He also could not fit into neither the Deaf nor hearing worlds. His mother's discouragement of the use of ASL made it much more difficult for him in and outside of school.
As for the older "professionals" having this attitude, it comes in all ages. I have seen it in younger people as well.
And unfortunately, as already stated before by others, that attitude is what is disabling.