Perhaps, and this is pure speculation, the damage was not so much from the heat, although if you saw some of the photos I have seen of the roundhouse fire and damage it wrought - wow - but rather the damage may have been from the roof and supporting beams, etc, falling on the boiler and denting the outer shell enough that the combination of the fire damage and the shell of the boiler may have been enough to condemn the locomotive - it may have been on its last leg anyhow and with the arrival of the dismals the fire gave the White Pass brass the excuse they needed to scrap the 72.
Just speculation.