My friends and clients occasionally dispute "whether mental illness exists" as an intellectual, scientific or policy issue. I think the question is confusing.
Certainly "mental illness" exists as a topic of discussion or investigation, and as a desciptive phenomenon, a prevalent and apparently useful metaphor, etc., in every Western public forum I have ever encuontered. Just as certainly, "mental illness" does not exist as any specific, proven disease or objectively measurable bio-physiological brain pathology. Honest, educated disputes regarding the existence of mental illness are therefore really about appropriate language and effective remedies.
But it's quite amazing how the overwhelming majority of people who talk about mental illness are either critically ignorant or dishonest. This especially includes people who make their living as mental health professionals, lawyers, judges and public policy makers, and those who claim to be mentally ill.
Most modern adults in Western societies would say that witchcraft and magic don't exist as they were defined and fought against for hundreds of years by the Inquisition and other religious and social institutions. But of course, there are Wiccans and stage performers today who are expert in the subjects of witchcraft and magic, who practice them daily and even make their living doing so.
Certainly "witches" do not exist to be burned at the stake, and just as certainly they do exist, in covens currently enjoying legal protection under the First Amendment's free exercise clause.
There's a great deal of work to do in the world. It is destructive to waste time confusing each other. We should know what we are actually disputing, and take personal responsibility for the understanding we impart, or fail to impart to others, every time we speak or write.
My argument is simply that medical treatment for spiritual, emotional or mental problems (i.e., psychiatry) is an extremely bad proposition. It should not be public policy, and anyone who seeks to enforce it overtly or covertly on others should be fought with ferocity.