Pick me, pick me....

From a technical perspective, you have to separate transmission failures into those that are due to wear-out, and which thus can be predicted in advance by oil sampling, clutch disk wear patterns, load testing, etc. We know from the X5 transmissions that have gone 200,000 miles that the X5 transmissions do have a reasonable life before being worn out. I would even go so far as to say that most if not all failures that we have seen are not due to wearing out, but rather random failures of smaller subcomponents. Those smaller components include plastic pieces in the torque converter, electrical sensors, actuators, software, and so on. The trouble with those sorts of failures is that they are far more random in nature, ie it is hard to draw a bell curve with a predicted reliability rate. (incidentally, it is also why there is such a debate on whether ever changing the transmission fluid really matters, given the nature of the failures that have been documented)

The likely commercial answer is that BMW expects transmissions to go at least 100,000 miles, since they offer extended warranties to that mileage, and such warranties do not appear to be heavily padded to cover the cost of any significant number of transmission failures. If I was the engineer involved in that program, I would have included a safety factor in that calculation, ie more than 100,000 miles.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue