I think you raise a very interesting question and I have strong sympathies for your conclusion. That having been said, there is a point at which it's possible to tell that someone is behaving irrationally in an absolute sense. By that, I mean that it can be determined that someone's behavior is systematically incompatible with achieving some important items among their desires (whatever those desires may be). I don't mean the desires they would have if something were done to get them out of the Silver Chair - I mean the desires they actually do have at the time, that they're not achieving because there is something interfering.

For instance, the drunk who does not believe he has a problem does not harbor a secret desire to destroy his family or die of cirrhosis at an early age or kill people in car accidents by driving under the influence. His belief that he does not have a problem is interfering with his avoidance of those nondesiderata. It's not that either his rational mind or the drinking have changed his desires re: kin estrangement/his liver/automobile accidents; it's that his desire for alcohol has hijacked his thought process to the point where he can't connect the excess of booze to the nondesiderata. If it's possible to excise the hijacking desire, other desires which are also important to the drunk will be in a better position to be fulfilled.

It's when we start being paternalistic, to achieve ends that the subject does not share at all and to save no one but the subject, that it becomes deeply murky territory.