I would suspect that even a dunce like me would know when to stop turning the crank, if the burrs jammed on a rock or other debris. That's the problem w/ a motorized grinder; by the time you know it's happening, the damage is done. I can't see that happening with a manual grinder.
My Compak has essentially the same burr set, and the factory says they're good for 3300lbs. Let's see, I go through a lb./week. Suppose I get groggy and up that to two lbs./week. That's only 31.7 years. Let's say the factory's 'extensive testing' is wrong by a factor of two; rats! I'll only get 16 years out of the burrs.