Look, if you were asked to help find the cure for cancer, but be subjected to testing materials that could cause you to almost certainly die or be disabled for the rest of your life, would you submit to that experiment? Billions and all who come after you get cured from cancer, you get to die in a vegetative state for the good of all humanity. Would you take that deal?
But what your proposing isn't what will happen to the Ba'ku, as we've seen in the S'ona. Without treatment they'd simply go back to their normal lifespan and live close to another century.
While people in the federation get to live forever, sounds kind of hypocritical.