Today's treatments, as frequently used, are as worthless and misguided as those exorcisms.
I'd rather have no loaf, than have no loaf and pay for it. These drugs have side effects, some of them very bad, and some of them permanent. (and doctors generally won't tell you about them in advance)
True, honest diagnosis first.THEN treatment.
What would be better is to avoid diagnosis labels (or use only more specific labels) until we can understand and test for the neurological conditions that lead to the symptoms of mental illness. We could differentiate "depression due to low $HORMONE" from "depression due to physically-damaged $STRUCTURE", and "low levels of $CHEMICAL" from "high levels of $OPPOSITE_CHEMICAL". From there, drug treatment can be centered on solving specific problems, rather than just brutally shoving the mind in a particular direction. Once the underlying neurology is a bit closer to "fixed", therapy can be more effective at helping the patient cope with their chronic condition.
This isn't academic, when-the-sun-burns-out stuff here. Hormonal, structural, neuro-transmitter (chemical), and opposing neurotransmitter problems can often be diagnosed today. They just frequently aren't because doctors are either too busy, lazy, or ignorant. It is very reasonable to believe we'll have even more of these things that we'll be able to test for in the near future.