"question is why we still have to shutt down after msg gone?"
Answer is: Because you may still have a fire that is just not triggering the fire loops... and/or, you may still be feeding a fire/potential fire by way of a fuel leak.... you have no way to know from the cockpit. please shut her down, pull the fire handle and blow at least one bottle!
Thanks mate.
Doing so and finding out later it wasn't necessary will be a lot easier to explain after the fact, than not doing so and finding out it was necessary.