Saying that mental illness is a myth is like saying that
birth defects and mental retardation are a myth. Mental illness is a chemical imbalance of the
brain, and it is very real. There have been
too many examples of mental illness throughout history for it to be a myth.
It isn't just brain chemistry that contributes to a person's state of mind. One psychiatrist found out during his research that both his genetics and brain chemistry matched that of many serial killers. While he wasn't known for being particularly empathetic by his colleagues he clearly wasn't an evil psychopath either. This suggests that biology does not guarantee there will be a mental illness and likely has more to do with broader personality characteristics rather than always causing symptoms whenever there is a certain chemistry.
And most people who get medicated will return to baseline if they don't also get talk therapy. In general people should be their natural selves and so research should be aimed more at helping people live happy, functional lives with the chemistry they have and not trying to change who they are on that fundamental of a level. Sometimes there's no other choice for managing symptoms but we should try to move towards non-drug therapies whenever possible.