Or it would have been indirectly prevented, in a world with the likes of JLA and the Spector and constant city-destroying crises, things like WTC would barely even register among the population when you have literal GODS coming down to save your ass. Not to mention with people like the JLA, JSA, Teen Titans, Titans, Suicide Squad, Checkmate and all the other forms of super-hero teams and legacy heroes, it would deter a lot of criminals and terrorists from acting. I would like to think that a lot of our current world problems doesn't existed or prevented by other superheroes in the DCU and Marvel. The reason why the life is still suck-ish there is because with the existence of heroes, the problems and fights just got bigger.