The factor you are missing in the middle is “language and culture developed specifically around this loss of hearing”. The identity isn’t built around lack of hearing, it’s built around a society that will be literally destroyed if the specific feature that mandates membership is eradicated.

As an analogy, how would you feel about a new mandate that all babies learn English as a first language?

I know several hearing people who are part of the deaf culture. They grew up with deaf parents or otherwise interact with that culture to join. I'm sure their kids will not be. Just like my grandpa was part of a German speaking culture that my dad never joined.

> As an analogy, how would you feel about a new mandate that all babies learn English as a first language?

I think it would be wonderful in its effects (I am not a native English speaker), but I don't like the "mandate" part.

As for the other point you are making - the language and culture were developed to work around physical issue of not hearing. Those who have learned the language can continue to use it after regaining hearing. I don't see why those who can hear couldn't learn it if they wanted to (e.g. to communicate with someone who decides to not pursue treatment for whatever reason). I also don't see why preserving something, that solves a problem that now has a better solution, is so important.