“What exactly were/are the Birchers wrong about?”
In that they trust their own common sense above that of anyone else.
Birthers don’t care who is against them, even if it’s their favorite conservative like Coulter or whoever. They simply trust their own common sense.
The anti-birthers can only hope to sway the birthers by forcing them to conform—to adopt a conformist, group-think mentality.
Beautiful summary.