What'll happen is not far from the consequences of skipping an engine oil change. It's not unusual for the ATF to collect fine metal particles over its lifespan, but having too much of it could ...

The ATF inside your car's automatic gearbox can reach 175 to 220°F in day-to-day driving. However, the temperature rises when towing, hauling, or driving with a whole load of passengers and cargo.

Like other car fluids, ATF should be changed from time to time. In manual cars, the transmission fluid is changed around every 30,000 to 60,000 miles.