Pretty easy to armchair QB. There are a hell,of a lot of people in the pictures I have seen. Most of those present are intent on something other than what is going on around them they're spectators after all.
I would venture to say going forward, yes, people will be more observant. At one time I went to a couple NASCAR races a year. Post 9/11 there was heightened security... But I believe a person could have brought a 5 gallon bucket of gas and a pallet of road flares past the gate. People ain't got time for all this crap. Watch your kids watch out for traffic watch out for rapists, pickpockets, panhandlers, terrorists with guns, terrorists with bombs terrorists with who knows what. People step in front of busses and get killed and a bus ain't hiding, trains hit cars with surprising regularity and the dangled things don't swerve at you.
Human nature favors the random terrorist.