It would require split-second coordination among a group of terrorists sitting in the first few rows to take advantage of this before the crew becomes alert and closes the door. And even that opportunity is easily removed (if it isn't already) with a simple curtain preventing passengers from seeing the cockpit door.

In any case, 9/11 was only possible because passengers had been previously instructed to passively cooperate with hijackings. Before the end of that day, they were ready to fight to the death instead.