I do think that people are gradually coming to discount the idea that there will be more terrorist attacks on the United States. I believe that the lack of attacks during times when the threat level is orange is part of the reason for this. However, I do not think that this is the only reason or even the main reason why people will tend to ignore future threats.
I would argue that the main reason why people will tend to ignore future threats is that they are no longer convinced that attacks are imminent within the United States. Since the 9/11 attacks, no further successful attacks of that sort have occurred in the US. The fact that we have gone almost 12 years without an attack makes us think that attacks are unlikely simply because we have short memories.
When Homeland Security raises the threat alert and nothing happens, it does help to make us complacent. It can be likened to the story of the boy who cried “wolf.” The more often we are warned of an attack without an attack actually happening, the less worried we will be. However, even without the orange alerts, we would still be likely to ignore future threats unless the government can be really specific about where and when it expects the attacks.