Point being that:
1) After staying below BMR for quite a while your body will adapt, your BMR will lower, thus hindering further weight loss.
2) There's a limit to how much energy you can get from fat.
This abstract shows some numbers, while
this article gives ~72 kcal/kg of fat mass/day as the upper limit of energy from fat. This is the absolute upper limit, which is extremely unhealthy for a normal person (severe caloric deficit). When your body has reached that limit it will consort to breaking down muscle tissue in your body, and as far as I know your body doesn't prioritize what gets broken down first. This will lead to a decrease in organ function, which in turn has nasty effects for your health.