Why make another bullseye for planes to crash into? In all realism, they'll just make a new terrorism target.
There are other things to blow up/target besides two towers like the Pentagon, which was also hit that same day on 9/11. Other targets could include the White House, some federal reserve, the mint, any military base, a subway, airport, basically anything that has a high concentration of people or is important to the US Government in is prone to some form of terrorist attack. It doesn't have to be a plane, it could be armed gunmen with a bomb, a Self Delete bomber, car bomb, a chemical/biological weapon, ect.
....what?
What the forget are you going on about, sonny? All the armies in the world can't stop an airliner crashing into a building at high speed, ya dingus.

Learn2SAM.