Well one potential outcome of a suit against YKK for a zipper failure might be that, in fact, the zipper itself didn't fail due to poor quality reasons - but instead the fabric shed and accumulated in the teeth wearing them down over time... Basically, with an assembled product, it's unreasonable to expect consumers to try and identify the actual fault of the design.

As an anecdote on this topic.... I recall having a long necked jacket as a kid where the zipper wore down heavily around the collar since the neck was so long that it ended up being too tall for normal day-wear - and thus a lot of unnecessary stress was put on the zipper mechanism when it was partially zipped up. After a few months the teeth had weakened in that area to the point where the zipper would frequently come off the tracks there. In this specific case fitting a jacket with a four inch collar caused a zipper failure - the zipper was probably cheaply made anyways but if the cut of the jacket had been different there likely wouldn't have been an issue.