Mechanical drive failure also depends on other factors, like how much the drive is being used: server environment, or desktop PC? And if desktop use, then are you blasting it with data over and over like for video editing, or just doing web surfing and word processing?
Also, that whole power saving scheme where your OS turns your hard drive on and off all the time is not a very good idea if you want the drive to last a long time.
Finally, heat plays a big role. Your ultra-thin-and-light laptop may look cool, but does the case get a bit too warm? If it does, then your drive's life will be shortened. Same for desktops. This is why if your desktop puter has an air filter, you'll want to keep it clean. And if you have one of those lovely uber-thin desktops (iMac or all-in-one), your hard drive will not last as long. Even if a hard drive only uses usually about 9W maximum, if there is no air circulation around it, it will get HOT. This is bad.
It's generally a good idea to upgrade to a bigger, newer harddrive every couple of years, anyway. For mechanical drives, the data density per platter increases each year, so whereas before your 1TB drive had 3 platters, now it might only have 1. That means less power usage, less heat, a much quieter drive, and often faster reads/writes (the curious can search for "areal density" for more info).
In short, keep backups, and don't wait until your drive dies. 3 years of normal use is the magic number (from experience, not the article).