Our metabolism slows down primarily because it takes more energy to keep a fat person alive than it does to keep a healthy person alive. After you lose 20% or more of your body weight you might start to notice this, it isn't a real concern. The effect is negligible and again notice that people taking drugs thet let them comply with a calorie deficit diet don't have to resort to excuses like this to explain a failure to lose weight.
The cause of a failure to lose weight with moderate calorie restriction is always a failure to maintain the calorie restriction aka cheating on the diet (because it is incredibly hard to do when you aren't used to it).
I think you know more than me on the subject. I have never cared about calories in my life and never really cared about my weight.
I lived abroad for a year and, when coming back home, started eating healthily (i.e. more greens and less reds, avoiding added salt). In about 3 months, I managed to reduce my weight by 10% - about 6kg. Although I have to admit, if anything, I wanted to keep those kilograms because it was helping me for weight-training lol. My objective was never to lose weight.
The cause of a failure to lose weight with moderate calorie restriction is always a failure to maintain the calorie restriction aka cheating on the diet (because it is incredibly hard to do when you aren't used to it).