The "chemical imbalance" theory is objectively wrong, but still useful in that it conveys the fact that mental disorders have physical causes. It's a purposeful simplification.
Indeed, a rebuttal [1] to that paper starts with:
> Moncrieff et al. report in a review of reviews that depression is not generally linked with serotonin deficiency. This is hardly news to neuropharmacologists, which Moncrieff et al. tacitly admits, as they justify their review by citing examples of laity and general practitioners believing depression is caused by a “chemical imbalance”, i.e., in serotonin. For instance, already in 1986 did Depue and Spoont point out that serotonin deficiency may not be a general cause of depression or other psychiatric illness. Further, that increasing extracellular serotonin—e.g., with selective serotonin reuptake inhibitors (SSRIs)—treats depression does not mean decreased serotonin causes depression.
I have a lot of trust in the science of medicine, but almost none in healthcare. It seems like the research is totally divorced from the medieval treatments I see doctors give all the time. "This is hardly news to neuropharmacologists" vs "laity and general practitioners believe ..." indeed.
Ultimately medication is prescribed based on evidence from clinical trials, whether or not the mechanism of action is fully understood. SSRIs work to help with depression in clinical trials when compared to placebo, so they get prescribed.