"Most ATF can withstand normal operating temperatures of around 200 degrees F for tens of thousands of miles. But if the temperature of the fluid rises above 220 degrees F the fluid starts to break down quickly. Above 300 degrees, fluid life is measured in hundreds, not thousands of miles. And above 400 degrees, the fluid can self-destruct in 20 to 30 minutes!"
Also above 175 degrees ATF life is cut in one half.
You may ask "why the radiator/ATF warmer?"
ATF (depending on individual Brookfield Viscosity rating) can become ineffective at extremely cold temperatures.
If in doubt, a transmission fluid temperature gauge sender can be TEE-ed off the transmission-to-radiator line.
Installing a Transmission Temperature Gauge