So, from the fact that underweight people are less healthy than neither-underweight-nor-overweight people it doesn’t follow that overweight people are also healthier than neither-underweight-nor-overweight people, and hence that there’s no good reason (other than signalling) for people to not want to be overweight, does it? (Or am I misunderstanding what the whole issue is about?)
Why are people assuming that the relationship between fat and health must be monotonic?
They aren’t. That would be crazy.
So, from the fact that underweight people are less healthy than neither-underweight-nor-overweight people it doesn’t follow that overweight people are also healthier than neither-underweight-nor-overweight people, and hence that there’s no good reason (other than signalling) for people to not want to be overweight, does it? (Or am I misunderstanding what the whole issue is about?)