There is a giant, 10 mile long stetch of runway with a 747 on the middle of it. The runway, however, is specially designed to move, much in the same way that a treadmill will move, in the opposite direction of the plane. Secondly, the treadmill is attached to a computer which will match the acceleration of the plane exactly and constantly, in the opposite direction.
What is the result of this experiment? Does the plane ever take off?
I'm not saying that the plane will not be moving in relation to the air. Of course the jets aren't what push the plane into flight directly. No one is saying that.
The jets are, however, what push the plane forward relative to the air surrounding it. The jets on a treadmill will do exactly that, they will push the plane forward in relation to the air surrounding it. The treadmill will be moving backwards, but that will simply cause the wheels to speed up.