Before September 11, 2001, the U.S. Air Force only had a few dozen fighter available for air defense of the United States. After September 11, a larger force of 250 aircraft and 11,000 personnel were devoted to continental air defense. Their mission was to shoot down any aircraft threatening to make another attack like those that occurred on September 11. But this has been incredibly expensive, costing $2.6 billion a year. Last year, there were about a thousand instances of suspicious aircraft activity reported to the air force, and in 200 cases, fighters were sent into the air to check it out. No aerial terrorists were found, and none have been detected since September 11, 2001.
Meanwhile, the air force is cutting its fighter force, because smart bombs require far fewer fighter-bombers. At the same time, it's become obvious that improved security in the aircraft and on the ground, and the likelihood that passengers would not allow their aircraft to be taken over, makes another September 11 type attack unlikely.
But there are still a lot of smaller, non-airliner, type aircraft out there that could be used in such attacks. These would still keep the fighters flying. But money and aircraft shortages are sure to result in fewer aircraft on the ground for those missions.