@OmSigDAVID,
To begin with, I'm not one of those folks who advocate the wholesale disarming of American citizens. The right of American citizens to bear arms is Constitutionally guaranteed, and should be honored except in limited cases, ie. Felony conviction for a violent offense, certified lunacy, and minors. However, for society to restrict ownership of fully automatic weapons, functional artillery, and military guided missiles is in my opinion reasonable, and proper.
Though the government may regulate who, where and how weapons may be possessed, that prohibition does not extend to private entities, like airlines or saloons. Even without government regulation, responsible businesses should exclude clients/patrons from their premises who are carrying firearms. The trend in the United States has for many years been to shift responsibility for such matters to the Federal government. That is the reality, as opposed to the philosophical ideal of the Constitution as we interpret it. The citizenry hasn't just willingly shifted the responsibility Federal government, it has demanded that the Washington regulate to some extent firearms.
The point of your post seems theoretical and idealistic. Simply put, "If citizen were always armed with handguns, then events like 9/11 would not be possible." Perhaps, there is a minor theme as well, something like, "unarmed citizens are passive citizens inviting attack".
Neither proposition stands up to scrutiny. In the 9/11 example for instance, the terrorists would have also been carrying firearms, so the risk of a bloody shootout in mid-air would have been extremely great. We know that the passengers of one aircraft were not passive, and by their heroism managed to frustrate the terrorist's mission though everyone aboard the aircraft died. Why weren't the passengers of the other aircraft more aggressive? Alas, we will never know. Perhaps some did make some effort, but were unsuccessful. The fact is that willingness to take mortal risks is uncommon, especially where that risk might endanger others. Arming Mr & Mrs Milketoast will not magically transform them into Audie Murphy, and that might be a very good thing.
Mere possession of a firearm does not make it effective, even if the person carrying that weapon is immune from assessing the risks of using it. Unless the firearm is in the hands of a practiced expert, the dangers of its possession often outweigh the advantages. When should the weapon be drawn? When should the trigger be pulled? Optimal ammunition varies from one shooting situation to the next, so what ammunition should be loaded? At what range should the target be engaged? How reliable and accurate is the shooter's skill? These and other questions are vital if a firearm is to be used in a combat situation effectively, and with minimizing the risk that innocent people will be harmed.
Police officers are expected to understand the laws regarding firearm use, to be expert in handling their weapons, and able to calmly assess and judge when the use of a firearm is justified. They fail one or more of those tests far more regularly than anyone would like. Police officers are schooled, sometimes as many as four times a year, in the legal strictures surrounding the use of firearms. Most professional law enforcement officers have to qualify with their service weapon monthly. Annual training/testing on Hogan's Alley/combat ranges demanding shoot/don't shoot judgments are common. Police policies dictate ammunition that is least likely to pose a danger beyond the intended target. Police carry their weapons almost continually, they spend individual and group time considering and planning how to use their weapons for a wide range of contingencies. Still, police screw ups occur because Police officers are also human. They still get excited in combat, they still make less than optimal judgments, they use their weapons sometimes inappropriately, and their weapons skills vary considerably.
Civilians carrying firearms almost without exception are poorly trained with their weapon, have some strange notions about the legal limitations on the use of deadly force, and are more likely to react emotionally and with poor judgment in combat situations. Armed citizens do successful protect themselves, others, and property every day, and those anecdotes are widely publicized. What is less known are the cases where a citizen's weapon is taken and turned on them with tragic results. There are no reports on how many citizens are in possession of weapons that are far beyond their skill capacity. How often does the citizen fire his weapon, once a year? At what ranges can the citizen be capable of putting every round in the ten ring? Armed and untrained citizens are a great danger to themselves and society. We individuals have the right to own weapons, but society also has the right to enact laws and strictures to reduce the risks that are associated with that right to own and bear arms.