It is the fact that no definition for mental illness exists that makes psychiatry so flexible. The choices are endless and limited only by the imagination of the psychiatrist. About the only group free of such charges are the psychiatrists themselves, although one could charge them with paranoia since they are so quick to accuse people of being mentally ill. There's even a category of mental illness for smokers. Since we don't know what a mental illness is, then anything can be used to cure it and anyone can be said to suffer from it.
Therapies can come and go as quickly as the illnesses.
In fact, as shown earlier, the two can reverse themselves. Today psychotherapy takes on many forms. About any activity the human mind can conceive of, the psychiatric mind can call therapeutic. Back in the days when Jesus was considered something of a therapist, instead of a patient, he met a madman whose major symptom was to run about nude. Today Jesus would be quite surprised, and Beau Brummel quite thrilled, to learn that nudity is a therapy.