Correct me if I'm wrong,

The lift is created by the difference in pressure on the top of the wing and the bottom. This difference in pressure is due to the air velocity which is caused by the jets and also when the plane moves.

But, we're speaking ideally, since the conveyor's velocity is always changing due to the change in the velocity of the plane to keep it stationary right?
So, ideally if there exists a jet that could make the air velocity fast enough to create sufficient lift without moving the plane it should be possible.

In my opinion this is not done cause it will be wasting too much energy.

I'm struggling to follow the discussion as well.
But I agree with as long as there is sufficient airflow across the wings it will take off.

The plane's relative velocity with respect to the conveyor belt will be twice the speed then if it is actually moving on the ground.