This is just wrong. Weight loss on the short scale is literally a function of the difference between calories in vs calories out. You can increase that difference by eating less, or exercising more. You can easily maintain your weight as you age by increasing the amount you exercise, it might involve more exercise than expected, but it is perfectly doable.

Fat is not satiating to everyone. If it works for you, good, but stop making sweeping generalizations.

Eating the correct foods will influence the amount of metabolic slowdown in response to calorie restriction, and also the percentage of weight lost from fat versus lean body mass. What exactly the correct foods are depends on your genetics, your activity level, and your gut microbiota, and are different for everyone. Pretty much the only constant is that a high fiber diet is better than a low fiber diet.