>>Apart from the cost, lets not forget the potential of confusions between the two systems, which may even cause accidents: Let's say the speed limit is 100 km/h, and the car driver goes at 100 mph instead (without looking at his modern instruments; using his sense of speed instead). Or an airplane pilot confusing feet with meters.

Nahh, that can't happen! Imagine that! Maybe in the future a spacecraft commander would get confused between the two measurement systems on a trip to Mars and crash into the planet by mistake! Oh, wait, that did happen!