I found these 3 studies relating BMI, body fat, and fat free mass to mortality.
They show that BMI vs mortality is a U-shaped curve. Mortality increases with increasing body fat and decreases increasing fat-free mass.
Two of the studies showed mortality decreases continuously with increasing fat free mass. One showed a reverse J-curve where mortality decreased to a point and then started to increase or level off. There's probably a point where you have too much fat-free mass but it's not something most people have to ever worry about.