Root Cause? (revised) I believe that the root cause of the failure of my clutch was the slipping of the non-turning throwout bearing against the constantly spinning pressure plate fingers. I have found that the throwout bearing is designed to be in constant contact with the pressure plate fingers. The throwout bearing was touching the fingers, but not with enough force to cause the bearing to rotate with the pressure plate. It did create enough contact to wear the tips of the fingers to thin nubs after 81,000 miles. The way to prevent this rubbing contact between the TOB and the PP fingers is to ensure that the clutch cable is properly tensioned. The Ford owner's manual recommends pulling up on the clutch pedal every 10,000 miles (I believe). It wouldn't hurt to check the cable tension more often.
Lesson learned? Ensure the clutch cable is properly tensioned by pulling up on the pedal with about 10 pounds of force approximately every oil change (3,000 miles).
Result? I hope a clutch system that last longer than 81,000 miles.