Try to understand the difference between heat and the temperature, It is not subtle.
Whether a planet cools down or heats up internally depends primarily on how much heat its surface can escape to space per unit of time.
If the amount of heat escaping into space is larger than the amount of heat delivered to the planetary surface from outside (mainly from the Sun) the planet cools down internally. This is because planetary interiors are sources of heat. If the amount of heat escaping into space is smaller than the amount of heat delivered to the planetary surface from outside - the planetary interior accumulates heat and its internal temperatures grow.
Now, the heat can escape from the planetary surface to space ONLY by radiation. The amount (the rate) of this heat escaping by radiation is proportional to the 4-th power of the surface temperature. Hence, all other things equal, if the temperature of the planetary surface doubles - the amount of heat radiated to space increases 16 times.
Try to find what is the average temperature on Venus, and on Earth, take the temperature ratio and calculate the 4-th power of this ratio. You will see that Venus radiates many times more heat to space than Earth does.
Hence, Venus interior may in fact be cooling down... To verify what is really going on you need to compare the amount of heat Venus receives from Sun with the amount of heat it radiates to space.
Earth's interior overheats, because NASA measurements prove that Earth's surface absorbs more Solar energy than it is able to radiate back. Ask NASA for data about Venus and you will know for sure if Venus cools down or heats up.
The fate of a planet can be determined by observing its surface from outside.