Well... another way of thinking about this is... if it costs 15,000 a year to treat, I wonder what percentage of those people could lose weight another way for less than $15,000 a year (e.g. free gym membership with classes, personal trainer, and $500 a month bonus if you show up twice a week)
We know the answer to that, though. Very few morbidly obese adults lose significant amounts of weight and keep it off long term, even with comprehensive lifestyle interventions like calorie counting, gym membership, and therapists. Even people who lose a lot of weight just gain it back at 1-2 kg a year. That's why there's an obesity epidemic.
