I suspect the number of people who would carry is smaller than you imagine. I own some handguns, have had a CCW in the past, and I wouldn't carry on an airplane. I'm not opposed to it, I just don't see a reason to do so. Same as I don't see a reason to carry in my eastside Seattle suburb neighborhood when walking the dogs: it's probably the smallest risk I'll face all day. (Granted, I walk two pit bulls so a hand gun seems kind of redundant. <g>)

My second, and largest objection is having a bunch of amateurs of varying abilities shooting in an environment where people are packed in like cattle. The singular time I have ever even considered drawing a handgun for defense, there were people in the area behind the potential target, with a non-zero chance of hitting them were I to miss. Too risky, found another way to resolve the issue. On an airplane? The background behind the target is almost guaranteed to have innocents. I might trust combat-experienced military personnel to take the shot, no way would I trust someone who has only put holes in paper at a shooting range with no adrenaline coursing through their veins (a group which would include myself).