Well actually, a Nissan Leaf is going to have 8 gears in the reduction drive. 4 in the differential, 2 to couple the differential to an intermediate shaft, and another 2 to couple that shaft to the motor itself. That being said, ATF does wear out over time, those gears are going to be adding finely powdered metal to the fluid and even just the shearing forces on the fluid literally breaks down the oil chemically. It'll certainly last a whole lot longer than engine oil but eventually that inspection is going to necessitate a replacement.