I've never in my life seen or even heard about a stair crapping out. Or specifically - I've never seen or heard about steps cracking, thus effectively rendering the stairs useless. A flight of stairs is something that's just there, and it provides an effective means of walking between levels. I've never even seen a ladder breaking down, and I've been on some pretty rackety ones myself. Including one situation whre I was standing in the top of two ancient ladders tied together with some rope, leaned at a too low angle against a tall building holding a can of paint and a brush, while maneuvering back and forth in order to reach as big an area as possible, because I wanted to minimize my time on top of a tall object, metal objects in hand, during a thunderstorm. Still the thing didn't break.
However, two weeks after we took over our new house, one step in the "main" stair cracks all the way through, and another one is getting pretty shaky.
Damn and double damn.