One can get into an 'intellectual' debate as to whether mental illness is a 'myth' but ultimately there are people whose functioning in one or more domains,be it occupational or social, can be impaired by the set of symptoms they present with.
These people need help be it medication and/or therapy to improve their functioning and for most debate about whether mental illness is a myth is a distant afterthought.
Such debates, though interesting, are not what helps most people,on a practical level, to function better .

It's all too easy to intellectualise while missing the fact that symptoms have very profound effects on people.