It's debatable whether a fine thread 3/8's is stronger than a 7/16 course thread. The 3/8 has more threads per inch and a shallower thread depth which is stronger and has more holding torque. One thing for sure, the 3/8 cost more so the switch was most likely for cost savings, not to mention how much easier it is on the assembly/machining lines to cut course threads. And since the threads are in the gear does it really matter at all????

I've heard a lot about axle instability but not much about a ring gear coming loose.