> Diets don't fail because they're inherently flawed and the system is more complex than we think it is. Diets fail because people fail.
The system includes the people. If the people "fail", that is relevant information, and should be used to evaluate the 'effectiveness' of the approach. The system is extremely complex, including as it does all of human psychology and physicality, and yet every one of these discussions is dogged by people like you, that think that the problem is just that people don't try hard enough.
Well sure they don't, that's also the reason they can't all bench 400 pounds - "hard enough" is the critical phrase here, and your language and attitude place all of the blame on the individual. It's a lot like explaining that depressed people are responsible for their own emotional state, and should just start exercising and cheer up - it's factually true, and yet also unhelpful, counterproductive, and insulting.