We aren't talking about physics. We are talking about a complex, open system. Something closer to macroeconomics. There is truth to what you are saying, but the certainty you are saying it with hides an ocean of complexity. Why are people excessively hungry? Does cutting calories linearly affect everyone the same way? What if we looked at the type of weight lost, is purely focusing on calories the best way to lose predominantly fat while maintaining muscle, organ tissue, and bones? Is focusing purely on calories effective for everyone (compliance) or does it set up some potentially disastrous consequences for some people?