Then they can just not get the treatment...?

Part of the concern is that if a cure for deafness becomes standard, then resources for the deaf community (e.g. sign language interpreters) may no longer be available for people who either cannot or choose not to get the treatment.

There's also an issue that, assuming they work similar in this regard to cochlear implants, the treatment has to be performed at a very young age before its possible for someone to consent or choose whether they want to be part of the deaf community.