In placing a fixed prosthetic restoration we are dealing with human tissue, human psychology, human behavior, and human host responce, all of which are variable in mostly out of the dentist's control. These factors plus others not mentioned influence the clinical life of a prothesis.
If health care ever gets to the point of warranting how long a patient wil live then maybe a 5 year warranty on dental prosthetics may be in line!
A 5 year warranty is an absurd notion.
Can you imagine the litigation and regulation that might result!