That was never true anyway, because the brain is a neurochemistry expert. It's all about regulating its chemistry on the basis of things other than chemical inputs to the body.

Have sex? See a lion? Running low on carbohydrates? Rejected by your tribe? Your brain is all over that.

So a priori, it's not at all obvious that even if you have a chemical imbalance, the right way to intervene is with chemistry. It might be, but unlike a failure of glucose regulation--where the ways to intervene are limited--with neurochemistry we have more potential options. (Just because we have options it doesn't mean that we know what to do. Or that the options can be targeted enough. But hey, SSRIs were never that targeted anyway....)

Furthermore, we still don't know whether depression is a chemical imbalance! We just know it's not widespread serotonin levels. Maybe it's highly targeted serotonin levels that we can't target accurately enough to measure. Maybe it's levels of something else.

So we should scratch our heads more, redouble our efforts to try to figure out what is going on, and remember that our brain is highly plastic and is good at neurochemistry but also that it is affected by chemistry.