Aside from what others have pointed out that it's perfectly okay for a scientific study to have limited scope and not try to solve the entire problem of how to lose weight and keep it off, which inherently requires following and monitoring people for a very long time, even the case you're describing here isn't necessarily all bad.

Given a person undergoes an intervention, succeeds in losing 40 pounds, then gains it all back over the next five years, that sounds like failure in a vacuum, but that means they spent five years not gaining more weight. If the non-intervention counterfactual is they would have ended up 40 pounds even heavier, then intervention is still a win. Yo-yo dieting with lifetime net zero progress is still better than steadily getting fatter for the rest of your life.