People that want to kill will find a way to kill. This is especially true of those who are willing to strap a bomb onto their bodies or willing to crash a plane into a building for their cause.
Each time will kill one "leader" another steps up to take his place. Every time they lose a key member of a group it just angers them and makes them more determined. Having said that terrorism is not something that is "new" it was around long before 9/11 took place. That was America's official invitation to the party.
As long as there are groups who are dissatisfied with their station in life and blame other nations for their unhappiness there will be terrorism.
Their leaders know they're never going to "win" or "take over" the world and suddenly have all 7 Billion of us fall in line with what they believe. They're content with disrupting the status quo and striking fear in the hearts of the effected areas they attack.
Terrorism is not a "conventional war" that can be won because there are lots of splinter terrorist groups who are not united. You can't reason with someone whose goal is to bring about the end of the world according to what they've been taught in their religion. The terrorists are not looking to gain land or wealth. There is nothing we can offer them short of our deaths that would pacify them.