I'll go so far as to say that lots of people do check the zipper when buying any clothes with a zipper, and usually put it back if it isn't YKK. I picked up this habit decades ago shopping with my mom and great aunt, but they acted like it was common knowledge. In years since I've met others who do the same. In my experience, most zippers that don't say YKK will break within a couple of years - backpacks or not.