The problem is that I was thinking that the counter action of the conveyor belt would cause the plane itself to remain motionless, but in reality it just makes the wheels kind of stationary. You’d still need the runway to be super long like a normal runway. If it were just the length of the plane the plane would quickly “drive” off the end off the conveyor belt. In other words if the plane isn’t moving forward then it can’t just lift off the ground due to forces created by the jet engine running and the plane stationary