Ha. I was thinking of "they" as the designers. "They" designed it to last long enough, but how do they know what long enough will be? Supposedly the original designed lifetime for the wonderful Austin-Healy 3000 was only 10 years. Now 60 years later people still drive and cherish them.

On the other hand there is the answer that Calvin's dad gave when Calvin asked how they knew the load capacity of a bridge - they drive heavier and heavier trucks over it until it collapses, then they weigh the truck and build another bridge exactly like the first one.

Um, I don't think I stress my aluminum handlebar so much. I'm just strong or heavy enough!

But I take your meaning.