I agree in principle.  In practice, the benefits of exercise can include an increase in the amount of calories you burn throughout the day while you're not exercising.  If you put on muscle mass, those muscles will require more calories to maintain.  If you do HIIT, your metabolism over the next day gets amped up.  Fitness also results from regular exercise and has many other health benefits.  So whie the diet is the most crucial part of the puzzle in terms of health and weight loss, exercise also contributes to health in a lasting way and gives you more of a buffer in terms of your daily energy burning.