"If we don't eradicate terrorists now they will keep coming and coming."
How exactly are you planning to eradicate terrorists? There are more terrorists in the world ready to sacrifice themselves for the sake of religion than you can ever kill. Even if Bin Laden is captured, what good is that going to do? Is the war going to end? No, someone else will fill his shoes. Terrorists don't have a nation, they don't care if Afghanistan gets wasted. They'll just move on to some other suitable location. The more you hunt them, the more people will join them. Or do you expect the civil victims of your country's actions to understand that it's all for their own good?
I don't know all the reasons why those terrorists do what they do, but I read that one (if not THE) reason is that the presense of USA troops near their "holy locations" in the Middle East pisses them off. I don't really see why USA can't just pull away from those areas. Backing off is damn much cheaper than fighting a war. Besides, USA is quite self-contained and doesn't really depend on those oil resources.