You don't need "everyone" to go armed...just enough to go armed to protect the others.
In any case, are you willing to let people die just because you see such an attack as having a low chance of happening? Do you think the people who died would agree with you? Or what about the more than 300 people that Jeanne Assan protected? She wasn't the only one armed at the time, but she was the only one who fired on the nut-job. Maybe those 300+ people would agree with you that the chances are too low? Or, maybe you don't care that more than 300 people were likely to die...as long as it keeps guns out of law-abiding people's hands?