I'm not sure this is correct. People who avoid obesity-related health issues often just get old enough to suffer from some other, even more expensive disease.

I'm not sure this is correct. People who avoid obesity-related health issues often just get old enough to suffer from some other, even more expensive disease. A cancer patient requires more expensive care than a typical diabetic or heart disease patient, and the prolonged costs of caring for someone suffering from dementia or Alzheimer's can be greater still. I'm not saying obesity is good for society (prolonging years of healthy life is a worthwhile goal on its own), but the impact on the healthcare system itself isn't necessarily so simple.