Lomapaseo,
My appologies if I atributed to you more than you wrote.
My main point is that a fire or other emergency situation is a very varied one where events can change extremely rapidly. The unbreached hull can appear relatively safe, however once a breach by fire or ingress of smoke occurs the situation can deteriorate extremely rapidly.
Evidence from Manchester shows that passengers were incapacitated with just 2 breaths. The second breath an uncontrollable reflex reaction by their bodies. Depending on where one is in relation to the smoke or fire, incapacitation or death can occur within seconds.
A panicing crowd with everyone fighting for their lives to get to an exit is not a good survival environment and eggress will be significantly slower than a calmer more orderly one.
It therefore makes sense to evacuate before actual fire and or smoke enters the cabin.
Your point that opening the doors will automatically cause an ingress of flames or smoke only holds true if the door is next to the flames.
However the cabin crew are trained to check for danger before opening a door. We can see this in the onboard video where a cabin crew can be seen blocking a door, outside of which there is clearly a fire.
(An exception would be a case like the rear right door at Manchester, however in this instance the main entry point for flames and smoke was the hull breach by the fire on the right hand side ).
Unless you are next to a usable exit survival time could be seconds not minutes, therefore maximum use of time by evacuating expeditiously is vital.