i think you guys are low-balling it too much. the average lifespan of a hard disk is probably 10-20 years i would say, depending on usage. i mean really, in schools and colleges they still run those old, ancient machines with the original hard disks from like 1990 ...at least at my schools they still do, and the machines are left on almost 24/7.
i'm sure they do not even have power management on for the HDD's. if they can make HDD that reliable back then, the new hard disks maybe just as good, although they had much lower RPM's back then.