"Mental illness is not a byproduct of modern society, its study and treatment is. "

If you have data on the incidence of depression in hunter gatherer societies we can compare to modern societies feel free to share it.

"Why is the rate of mental illness going up? It's because the stigma over mental healthcare is dropping rapidly and more people are getting formally diagnosed."

So in your mind the functioning of a society has no impact on the rate of mental health problems?