"But the overall average life span is falling now because of weight related preventable illness"
It isn't just weight related, but diet and lifestyle related.
I attended a health seminar, Consultant led. Every clinic sees 40% correct weight people, who think they can overload on carbs/sugar, or not exercise, because they aren't carrying excess weight.
Which is the danger of focusing on weight.
Even portion size. It needs to be about nutritional content.
I eat a greater quantity of food when I'm eating healthier. Plates full of lentil/veg curry, with no carbs rice vs a small portion of chips or a few slices of pizza.
The reason obesity isn't good for us, isn't because we are bigger, it's because of the lifestyle that has made us bigger.
Follow a similar lifestyle and are thin, equals the same health problems.
Plenty of other groups, who are the correct weight but drink over the recommended amount, recreational drug users etc are filling up other clinics.