Eating less calories than you burn throughout the day or week causes you to lose weight, that will remain true for every single person that does this unless they have a medical condition that causes weight gain, water retention, etc. That's a simple fact. The intricacies of how much is lost, what other factors such as weight lifting or cardio and their impacts are, etc are more complicated. Some people can eat less and lose a ton of weight, while others can eat less and lose weight more gradually, and others lose it more slowly.
Your argument that eating less doesn't solve the problem in the long run only applies to people who eat less and do nothing else, possibly for the rest of their lives since age and metabolism will start to play a factor. Someone that continues to eat less and work out, get to their goal weight, then gradually put on muscle which helps burn more fat, they are then able to up their caloric intake, in fact it's required to do so to gain more muscle. If that's their goal. But if you want to lose weight, yes, all you need to do is eat less calories. I'm not exactly sure what your point is, since it's a pretty basic rule to personal training someone or being a dietician. "How much do you eat?" "oh I eat about 3000 calories a day and don't work out" yeah, that's why you're fat. If you did nothing all day and remained sedentary, but ate 1500-2000 calories a day (a 500 to 1000 calorie deficit to maintain your current weight for the average male), you would lose weight. Plain and simple. And it doesn't matter what foods you ate, so long as you kept at that number.
It's also not cut and dry in the sense that whatever calories are reported on the back of packages or nutritional packets are often estimates. Those 120 calorie cookies you eat? 1 of the cookies you just ate could have been more than the 120 printed on the box, and the next one could be less.
Ketogenic diets might not be fads, but they aren't without their complications. They help you burn fat and lose weight, but they also have a negative impact on high intensity workouts. So it's not advisable for someone that's looking to bulk up or perform an activity that is high intensity (think tour de france, high level hockey, football, etc) to be on the ketogenic diet due to its negative impact on intramuscular glycogen and muscle mass. Your body NEEDS carbohydrates to perform those tasks at maximum efficiency.