Type 1 represents communities in which two or more languages are used, but where almost everyone is monolingual. The example given in the chapter is Belgium, which officially is multilingual, but where most people either use Dutch or French or German. Theoretically, the situation sketched in the question could be possible. But for deaf people to be able to use only a sign language in their daily lives (i.e. to be monolingual), the majority of the hearing community has to be proficient in the sign language since deaf people are always in interaction with hearing people. This is a rather unlikely scenario. Also, internationalization, the advance of Cochlear Implants, and the progress in genetic research work against this. Because the proportion of deaf people compared to hearing people in any community is always smaller, and smaller groups usually adopt to the majority, it is very unlikely that a type I community would arise.