Take a body of air. It can be doing whatever you like within itself. If it is in equilibrium with its surroundings (not rising, falling or moving) then it is no more than a hot air balloon with no envelope... In the end, the total forces on it must be zero. All of the downwards (weight) forces must balance all of the upwards forces (caused by pressure on the ground. If this is not true then it must be accelerating upwards or downwards (outwards or inwards).
That's a big if, around which the entire disagreement pretty much revolves.
The cold air flowing downslope in a katabatic wind isn't in equilibrium with its surroundings. That's why the katabatic wind happens.