I have a so called autoimmune disease. I have gotten dramatically healthier by making dietary and lifestyle changes.
My experience of being diagnosed with an autoimmune disease is that it is a good excuse for doctors to wash their hands of responsibility for getting you well. They blame your defective body and their goal is "symptom management."
Using your cancer example, it would be like not trying to rid your body of cancer and just trying to slow the growth of the tumors instead. People would balk at being told we aren't even going to try to cure you of cancer, we are just going to manage your symptoms until it kills you. But this is the gist of what patients are routinely told if their condition is classified as an autoimmune disorder.
Furthermore, my experience of my condition does not fit with the idea that my body simply attacks itself for no apparent reason. My experience fits much more closely with the idea that my body is the battleground and it takes collateral damage in the fight.
I believe when the body attacks some part of itself, that body part likely carries infection or otherwise has had something go so wrong that treating the entire area as The Enemy makes sense at this time. That state can be reversed. If it is reversible, no, the body has not simply gone insane and started randomly lashing out at itself. Its actions are logical and purposeful.