There are two different scenarios in which someone may attack a developed city in this way and while the initial hysterical-phase reaction is likely to be similar in both cases, they are not the same:
1. Some small demented group of crazied (aum whatever or suchlike) gets it into their head to do something spectacular. They kill many people, they get killed, their associates and known members get tracked down and killed or imprisoned, billions or trillions get wasted on new "Mall security" or some such, but its not really a repeatable and constant threat.
2. Some group with a real agenda, a real organization, a real ideology, attacks a developed city because their grievance with country X has reached criticality. The most likely suspect for such an act in the years to come is the jihadis. But even they cannot do this again and again. After the first attack or the third, their goose will be cooked. A sustained campaign needs a base, needs an organization, needs a pipeline of volunteers and trainers and financiers. How long before Pakistan is forced to stop any and all connection with ANY such business (or to die trying to cut them off)? I dont see how it can become a sustained and "normal" threat in advanced countries.
Its a different story for corrupt and incompetent regional powers. THEY should worry.