It just plain never was a theory of the brain. It wasn't even a hypothesis but more an analogy. AD's can influence the outflow or retention of neurotransmitters (natural chemicals of the body, including the brain). It's metaphoric that the chemicals may be pushed into a better balance that helps the person cope with emotions and other things influenced by biochemistry - very complex things that are far from fully understood. Drugs were developed, but if you read about them, it's not even fully understood how they work!

The main problem with the over-reliance on the "chemical balance" concept is that so many people seem to assume that chemicals come first and everything else comes second. However, physical or behavioral things as well as emotions can all influence biochemistry and "chemical balance" just as much as in the opposite direction, but things don't work like clockwork on all people. People are not the same chemically, genetically, etc., and are very complex systems poorly understood.