On the surface that seems like a solid argument - but I think there are some key differences when comparing a situation where a cancer patient refuses surgery and somebody with mental illness refuses help getting their lives in shape. For a start surgery is clearly a much more invasive and risky operation, and even if successful at removing the cancer, can leave the patient in a physically weakened/dependent state for some period of time. It also clearly requires highly-trained specialists to perform the operation and can only be done in controlled environs (operating theatre etc.). If there were any suggestion this were true for providing assistance for the severely mentally ill, then I'd be more inclined to agree with you that there's no good justification for imposing such treatment without their express consent. As it is, I certainly agree that there needs to be a strong system of checks and balances for any such scenario, including time-limits over how long it's legal to hold somebody against their will. As it is, we don't let dementia patients simply do whatever they like even if they initially consented to entering a care facility, but then no longer want to stay there (which is not uncommon). Whether it's a duty-of-care argument, or simply that doing so would result in too much risk to others, at some point we have to accept even grown adults are no longer capable of rational/informed consent, and we do actually have doctors trained in making such diagnoses.