Muscle is the currency
Aggressive deficits pull amino acids from muscle along with fat, and muscle is where resting burn lives and where body composition is decided. Losing 2 lb/week might be half muscle; losing 0.7 lb/week with adequate protein and resistance training is nearly all fat. Same scale change, utterly different body.
Adaptation punishes heroics
Deep deficits trigger adaptive thermogenesis — NEAT drops, hunger hormones climb, and the body defends its setpoint with 300–500 kcal of extra efficiency. A 1,000 kcal crash becomes a maintenance diet within months; a 400 kcal deficit keeps working. The rate limit is not timidity — it is engineering around the adaptation curve.
Adherence is the whole game
The best diet is the one sustained for the duration, and duration is measured in seasons. Plans that survive a bad week, a holiday, a work crisis — those finish. Slower paces build in slack: one indulgent day is a rounding error at 0.7 lb/week and a full week's progress erased at 2.5. Design for the bad week and the math forgives you.