There's where you've got force and thrust mixed up. The conveyor is only effective on the wheels of the aircraft. How can the conveyor counteract any thrust against the air around the aircraft?

As I said in an earlier post, the wheelbrakes on the aircraft could be locked on, but the engines (presumably on the wings) could be at full throttle, pushing the AIR. At this time, the aircraft has both zero airspeed and zero groundspeed. (The conveyor is also at zero).

Now imagine that you tie a big cable to the back of the aircraft so that it really can't go anywhere. Now imagine that someone walks up to the wheels of the aircraft and then tries to turn the tires by hand (simulating what the conveyor would interpret as forward motion). Let's say that they could somehow get those tires spinning at 100mph, so that the conveyor is also going 100mph, but in the opposite direction.

Now, you've got the tires spinning at 100mph, the conveyor moving at -100mph, and the aircraft, engines still at full throttle, pushing on the air, but the aircraft isn't going anywhere because it's secured by a rope.

What is counteracting the engine thrust against the air?

The answer: NOTHING is counteracting the thrust! Therefore, the aircraft moves forward through the fluid of air. As it does so, the tires accelerate, the conveyor accelerates negatively, but the surrounding air does not accelarate negatively, so the aircraft can build up enough airspeed to take off.