Unfortunately snakecharma, no one on the aircraft has perfect information.
You know there is an indication of fire. There may be a smell.
You have absolutely no way of knowing the severity or likely course of the fire once the aircraft is stationary.
Therefore you MUST invoke the precautionary principle and evacuate.
To put that another way, the situation you describe is exactly analogous to the question about going around: Its better to be asked why you went around rather than why you didn't....
..Speaking as an incinerator of a C172, you have no way of knowing what is going to happen.