Very few people buy a bike and ride it until the frame breaks from normal wear and tear. It happens, I'm sure, but that is rare. Frames get damaged falling over in the garage, crashes, hitting holes or obstacles in the road, etc, and frames get replaced for something newer. The question for me about a frame is how well can it take "normal" abuse - an inexperienced mechanic, improperly loaded onto a stand or bike rack, getting knocked over or surviving crashes.