The counter-argument is that in some circumstances, is no certainty that a fire will not break out at some arbitrary time after the plane has landed and come to

The counter-argument is that in some circumstances, is no certainty that a fire will not break out at some arbitrary time after the plane has landed and come to a stop. And if one does break out, there is no certainty around how long there is between ignition and Shit Going Badly, so the prevailing result is to get the plane stationary, get everyone off as quickly as possible, and then assess the situation. edit for clarity: I understand the issue you face regarding your medication being trapped on the plane and believe airlines should indeed have a duty of care to ensure you have rapid access to replacement medication at their expense, but I'm specifically addressing why they have the policy of telling people to not grab carry-on items during evacuation.