The number of studies that
you're personally aware of may be zero. I'm not confident in your knowledge of the state of research when you missed the fact that they didn't even track diet in the study you linked to prove
your thesis.
There are thousands of examples online of people who've lost weight and maintained that weight loss, not to mention the thousands of people who manipulate their weight at will to compete in events with weight classes.
Studies about diet and exercise are usually poorly designed and poorly funded, so they don't always accurately reflect reality and the existence of thousands of successes isn't invalidated by a poorly run study. Nor is our knowledge of basic physiology.
Rule of thumb: any study that relies on self-reporting diets is basically useless. It's not about lying. People are just really bad at measurement unless they explicitly measure every gram of food that enters their mouth. Intentions don't affect the quality of an implementation if you don't know how to implement something successfully. People who are underweight tend to eat less than they think and people who are overweight tend to eat more than they think.
Note that studies in which the number of calories a person consumes are tracked by the researchers leave no ambiguity in the direct relationship between the number of calories a person consumes and how their weight changes.
Distilling these issues into one of willpower is just incredibly misguided IMO and reflective more of the views of journalists who struggle with their weight and pass on the results of poorly designed studies. I addressed what I thought was a better way of looking at these issues in another comment: https://news.ycombinator.com/item?id=13827878
It's not as though Americans have "lost willpower" over the course of a couple generations. We just started eating more and moving less.
Why we started eating more isn't a mystery. Agriculture and food companies started processing food to be less satiating and more addictive by adding sugar and fat to sell more (a massive simplification of history, admittedly). Obesity wasn't an epidemic back when we were eating whole foods since they're so much more satiating and it's difficult to maintain a caloric surplus on a diet of lentils and vegetables or whatever.
As countries like France, China, Japan, and others who have more recently maintained their normal weights on a population level get introduced to these processed Western foods, they too are seeing their rates of obesity, heart disease, and diabetes go up.