I have been practicing for 19 years.  During seven of those years I worked as a subsitute, and saw many differnet styles of dentistry.  I have seen bridges last (patient reported) for thirty years.  I have seen (in my opinion) poor margins on some crown and bridge last for many years, because the patients took excellent care of  them.  I have also seen the opposite.  I have seen excellent margins (in my opinion) on some crown and bridge fail, because the patient did not take care of it as they should have.  They did not come in for regular dental hygiene appointments, as well as not regularly flossing, and what about using extra fluoride?  How can those gauges  be impliemented?  I myself had a crown with excellent  margins; I practice exellent hygiene, yet had a crown fail.  It happens.  Most dentists already offer a 5 year warranty on their work provided that patient practices regular preventive oral hygiene and has regular 6 month (or 3 month if periodontally involved) prophylaxis appointments.  I don't believe a blanket statement of  requiring dentists to give a five-year warranty on crowns and bridges to ensure work is durable and thorough, there are just too many variables that are beyond the scope of the dentists ablity to control.