It does little good to point out to true believers that most of their ailments would have cured themselves and gone away had they done nothing at all to treat themselves. One's tooth pain or facial pimples may have gone away after drinking a cup of one's golden fluid, but that is not strong evidence that there was any causal connection between two events. And you waste your breath trying to get a true believer to consider the possibility that to begin with they were misdiagnosed by their alternative practitioner. Thus they were never cured because there was never anything wrong with them. Many maladies have a behavioral component, often connected to the subjective evaluation of one's pain.
Belief affects behavior and behavior affects the body.
Furthermore, acting in a healthy way could cause objective, measurable improvements. In such cases, it would be misleading to say that the therapy was useless. However, it was not the therapy as such that led to improvement, but one's belief in the therapy. It might make all the difference in the world. First, there may not have been a cure after all.