I still don't get where you're coming from. You seem to imply that cardio and cutting calories via diet will decrease your metabolism and you won't lose weight anymore. There needs to be a reason for your body to need more/less fuel, it doesn't just change magically. I mean, obviously if you drop a ton of weight via calorie deficit you will need to make adjustments, you body needs less fuel when it's half the size. If you go back to eating the same as before you will of course gain the weight back.
If you cut caloric consumption and increase your expenditure via exercise routines, your body has multiple options, it can:
- Catabolize subcutaneous fat;
- Catabolize organ fat;
- Catabolize muscle;
- Catabolize organ and bone tissue;
- Decrease metabolism, by decreasing heart rate, body temperature, brain activity, etc;
- Decrease body activity by making you more tired, etc.
All of these are observed to occur in nature, in both people and in animals, multiple times over. Nature has many different ways of respecting the first law of thermodynamics.
There are in fact many reasons why the body may respond in different ways. The process of converting fat into energy to maintain metabolic activity is one specific pathway that involves a lot of different hormones and enzymes. They might not all be available in the right amounts at any given time. Moreover, fat is not an inert organ, it's surrounded by extra skin, nerve endings, blood vessels, etc which eventually need to be catabolized as well. It's a complex process for your body. It's not equivalent to simply using up a battery. That's why your body always runs through its glucose reserve first, prior too catabolizing adipose fat, as that's a simpler process.
The idea that hormones have no effect on body composition is quite frankly purely ideological in its irrationality. Nobody actually believes it. For example, it is well known by doctors that every patient taking thyroid medication, insulin medication, prednisone, etc. is at risk of arbitrarily adjusting their medication in order to lose weight. It's well known by the patients that their body composition will change effortlessly if they do so -- but the doctors know that there can be a steep physiological cost. Similarly, women who going through menopause also report that their weight gain is independent of their diet and exercise regimen. Hormones influence the body's composition, and factors such as metabolism and appetite respond accordingly.