"Unlike other branches of modern medicine which use biologically-based tests of disease, psychiatry still relies on patient reports and physician observations".
That is true. But unlike other branches of medicine, psychiatry doesn't try to fix people that aren't struggling. We go to the doctor, who then tells us our BP is too high and we need to exercise more. We don't go to our psychiatrist for such checkups. A doctor knows what good and bad blood pressure look like, and will seek to move us from bad to good. Today, psychiatrists rarely ever examine, let alone treat, people until their mental state is negatively impacting their lives. They treat the clinical case. They don't go around telling people they are sick in the way medical doctors look at blood pressure. A brain scan, a "biologically-based test" opens the door to treating people who aren't yet a clinical case. I am sure that such scans would aid in the diagnosis and understanding of mental illness. But by expanding the diagnostic process, these scans would also open up a huge new market for the psychiatric treatment of non-clinical cases.