Overfitting may be a special case of Goodhart's Law, but I don't think Goodhart's Law in general is the same as overfitting, so I don't think the conclusion is

Overfitting may be a special case of Goodhart's Law, but I don't think Goodhart's Law in general is the same as overfitting, so I don't think the conclusion is well-supported supported in general; there may be plenty of instances of proxy measures that do not have issues. I'll also quibble with the example of obesity: the proxy isn't nutrient-rich good, but rather the evaluation function of human taste buds (e.g. sugar detection). The problem is the abundance of food that is very nutrient-poor but stimulating to taste buds. If the food that's widely available were nutrient-rich, it's questionable whether we would have an obesity epidemic.