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.
Most of the older folks to who get cancer and age related neurodegenerative disease also seem to succumb to age related diabetes symptoms and poor insulin.
These things aren't separate-able as cleanly as you suggest.
See when I talk about threats to other, I am talking about direct, articulable threats, not second or third order events that may have an impact under some circumstances
When I say the government has an ethical mandate to protect me from others, I mean protect me from physically assaulting me, from stealing my property, etc.
Not generalized adverse effects that may happen latter down the road if enough other people also happen to do the same thing.
"The law is the organization of the natural right of lawful defense. It is the substitution of a common force for individual forces. And this common force is to do only what the individual forces have a natural and lawful right to do: to protect persons, liberties, and properties; to maintain the right of each, and to cause justice to reign over us all. " -- Frédéric Bastiat