Then the temperature should increase with altitude, instead of decreasing.

The laws of thermodynamics favor hotter matter achieving a higher altitude, due to reduced density from the expansion of the hotter gas. If the heat source is from above, this will increase the effect, incontrovertibly dictating that the highest temperatures will be found at the highest altitudes. Contrary to this, the Venutian atmosphere's temperature is the greatest at the surface of the planet. (See Figure 1.)