Your metabolism is a little bit elastic, it is most definitely not 100% elastic, which is what you argued.
The vast majority of people will definitely lose weight if they cut a third of their intake, all else being equal.
You're arguing about a marginal effect, and arguing against the primary effect. The primary effect is that eating more than burned is what caused the weight gain, and eating less than burned will cause weight loss. That's true regardless of metabolic elasticity, but even so changes in metabolic rate account for only a small fraction of changes in intake.
The vast majority of people will definitely lose weight if they cut a third of their intake, all else being equal.
You're arguing about a marginal effect, and arguing against the primary effect. The primary effect is that eating more than burned is what caused the weight gain, and eating less than burned will cause weight loss. That's true regardless of metabolic elasticity, but even so changes in metabolic rate account for only a small fraction of changes in intake.