Consider Vx and Vy, one is over distance (angle) and one is over time (rate of climb).
Now take that same concept and consider a descent.
With a fire your main concern is time, the average hull loss as a result of fire has occurred shockingly fast.
Even if Mmo / Vmo may give you a shallower descent it’s at a higher speed therefore allows you to cover more distance then a descent with gear and flap hanging out at a much lower speed.
Its much better to have an aircraft safely on the ground that may have exceeded speed limitations then a hull loss in the air.
According to Boeing overspending is a much lesser issue then over stressing the airframe.