The first problem is in knowing whether being "cured" would make them unhappier or otherwise, and how one would find out. Then there is the issue of autonomy, which we value even if it makes someone miserable, or more miserable than they might be otherwise. After the "cure", presumably the individual has greater ability to take their own decisions etc. and this can lead to problems of one sort or another. It is the old question of whether it would be better to be a contented pig or an unhappy Socrates, although it seems to me that Socrates often appeared to be rather cheerful, and pigs do occasionally look rather mournful, especially when something tasty is out of reach.
Your question does raise the issue of how important happiness should be to us, as opposed to other valuable human aims such as freedom of choice, knowledge, a variety of experence and so on. What sorts of life are worth living, and more worth living than alternatives? These are the issues that are relevant here.