I'm surprised you discount all psychogenic illnesses - these do exist in delusional disorders (psychotic) and seem to exist in non psychotic disorders from an unknown etiology (conversion, irrationality, etc) - i.e. non epileptic seizures. I reject flawed psychogenic models and studies, but I have an impossible time believing no somatic diseases actually exist.
There is a difference between brain derived
symptoms and mind derived disorders or diseases.
I have no issues with research into psychogenic illnesses, in principle. Yet, in the history of medicine,
not one has been proved. None should be taken seriously, as the evidence is much much worse than for CFS, and is right up there with tubercular personality, personality created heart disease, or women getting breast cancer because, well, they're women.
Non epileptic seizures have been
presumed to be psychogenic.
Irrationality ... its a human trait, not a disorder. Most of us can learn to be rational, but lack training. Just as we can learn mathematics or many other things.
Somatic diseases are bodily diseases, like HIV/AIDS, cancer or a heart attack.
Psychosomatic disease was originally, prior to Freud, considered to be the study of how the mind and disease interact. At the time it was a valid research area. Its questionable that mind is still a valid medical concept though.
With one caveat I mention from time to time, and there are caveats on that caveat, there is no proof that even one mental disorder exists. We do know that brain disorders exist. We do know that informational "disorders" exist, as in cult brainwashing. But what about supporting your local football team, being a patriot, or voting for one party all the time, or belonging to one religion, and here I include athiesm? These are cultural and social things, and its not a great idea to medicalize them. The Soviet Union did that with people who disagreed with its politics.
Further, it has been recognized (widely in fact, and I think there is a disclaimer inside the DSM itself) that not one single disorder category in the DSM is really validated. Even depression. We know something is wrong, lots of evidence of that, but not what is wrong nor if there is a valid discrete category.
Now I do think that some categories in the DSM will prove to be valid, but will that be 1%, 10%, 90% or 99%? Depression, for example, is a range of similar symptoms that seem to apply to many diseases and injuries. The symptom is real, but we are kidding ourselves that there is a discrete diagnostic category with good validity, representing a disorder or disease. If follows from there that the research to find a cure hinges on finding mechanisms. It also will not be one cure, but a range. So far we have none. Hence depression is just a symptom, or range of symptoms, and its a category mistake to call it a disorder or other disease entity. We treat it palliatively. It often resolves on its own. Its a lot like the Oxford Definition of CFS, a hodgepodge of different things brought in under an umbrella label.
I should add that there is strong and growing evidence that schizophrenia and Alheimers are in the process of validation. They are however brain disorders, involving the immune system and metabolism respectively, though with many other complicated processes still being researched. I think it likely most mental disorders will be renamed as brain disorders in time.
Now brain disorders can induce disordered cognition and emotion. Just look at brain injury for example.
Emotional events can also trigger changes in behaviours. Its the mechanisms that are under question.
Until we have evidence that a psychogenic disorder actually exists, and I mean tangible experimental evidence, thoroughly tested and not just supported by poor quality research designed to amass evidence of what
might be, we should regard it like the Yeti, or aliens buzzing the White House in flying saucers, or magic, or demonic possession.
Psychogenic disorders, as a diagnostic label, exist because some people thought they existed, it was a convenient label, and definitions were written up in the DSM, which was then marketed. The science behind these is very very limited, and there is nothing definitive at all.
Psychogenic disorders might exist, as a research possibility, and people who are diagnosed with these things deserve treatment. However they deserve appropriate treatment, with awareness the diagnostic categories are unstable, and that the claims to causal mechanism are based on theory.
People with "mental" disorders deserve better. Much better. They deserve cures, not palliative treatments.