Expertise was not the issue at all; just mindset.
The people on those planes expected to be held hostage and hoped to recover their freedom. They had an image of an outcome where few died.
As soon as the hijackers' intentions were known, action was taken and resulted in the deaths of the hijackers. Since all aboard were killed, there is no chance that people unskilled in the use of firearms would have had a poorer outcome.
Virtually every courier carrying cash to load ATMs is armed and prepared to use that firearm to protect the cash. People everywhere, including on airlines, deserve the same protection.
My point was that one gun among the passengers was not enough. There needed to be ten or twenty or thirty for the odds to be in their favor of taking out the terrorists.
Everyone comes to the realization that death is imminent, and action is necessary, at different points.
Someone who is confident in their ability to change the situation will take action a lot sooner than someone who can't remember where the safety is on the weapon they have in their holster or purse.