The NAS itself will likely outlive the drives inside, just the nature of things. Hard drives follow a sort of curve when it comes to failure, most fail either immediately or in a few 10000 hours of run time. Other factors include the drives being too hot, the amount of hard power events, and vibration.
Lots of info on drive failure can be found on Backblaze's Drive stat page. I know you have shucked drives, these are likely white label WD Red drives which are close to the 14TB WD drive backblaze uses.