Waterfall model software process model was derived from general system engineering process and it is so called as the stages go top to bottom and while one stage of the process is complete it moves to the next stage and not supposed to come back to any previous stage.
Before we learn about why software does not “wear out”, lets learn about the state “wear out”. When something is no longer of any use, it reaches the “wear out” state. That is, it can not perform the function it was built for. For example, a printer reaches “wear out” state it can’t print anymore. This doesn’t include the recycling options. One make use of a dead printer to do anything else but printing.