The problem with attempting to place a warranty on major dental work is that we as dentists don't work in a static environment.  Many factors can change the oral condition, some even beyond the patient's control much less the dentist's .Systemic diseases, medications as well as a patient's physical and mental limitations can all produce an oral environment more conducive to decay.  And the patient's responsiblity can not be negated.  Keeping up with regular dental visits to maintain proper care is as important if not more so than the skill of the dentist.