but there are deaf people who do not sign and who are happy and well adjusted individuals
Not quite true, For one thing, I grew up having deaf friends that were cuers and not signers because some of their parents didn't take the effort to learn sign language in order to communicate and perhaps their parents didn't provide all the tools that are out there and instead they made their children grew up oral...Most of them learn sign language later on, when some of them didn't...and I even heard them saying " I wish I knew sign language when I was younger "