For most people weight creeps up on them slowly. Losing a lot of weight quickly does require radical changes to diet, but not putting on the pounds in the first place is generally going to be a matter of burning an extra few hundred calories a day.

Also, just not being sedentary all day is associated with a ton of positive health outcomes (possibly mental even more than physical) independent of body fat percentage. So it’s worth it all on its own.

Of course! Nobody can deny the panacea that is exercise. But it's still a widely misunderstood concept. Throughout many of these comments, people are claiming that the remedy to obesity is more exercise, but I hold that indeed a much larger component of combatting obesity is diet, not exercise.

I’d say on a population level exercise (as in more active, less sedentary lifestyles) will be a large factor. Though on an individual level you are right that diet will be the big thing.

Though population level there is also a lot of food policy and incentive structures we could alter to improve diets in general as well.