Clinical studies on the longevity of dental restorations has shown that the two primary reasons for having to replace a restoration (crowns and bridges, fillings included) are new tooth decay that occurs in the remaining tooth under the restoration, and fracture of the restoration. Fractures tend to take place earlier in the life of the restoration and new tooth decay tends to occur later in the life of the restoration. Tooth decay is the main reason for having to replace a dental restoration with fracture a distance second reason. Tooth decay in a restored tooth is not something the dentists can control since it depends on risk factors that exist in the patient such as past history of tooth decay, strength of the bacteria present in the mouth, dietary use of sugars, saliva amount and content, oral hygiene practices, and use of fluoride toothpaste and rinses. Dentists should talk to patients about these risk factors prior to restoring teeth or replacing missing teeth with bridges so the patient can make an informed decision to proceed with the restorations but there is no way to predict or guarantee when or if the restored tooth may develop new tooth decay.
I have published two review papers that looked at the scientific literature related to the relationship between the fit of restorations to the teeth and the risk of tooth decay. The science does not support a relationship. Dentist should stirve to make well fitting restorations but a poor fit does cause tooth decay. Tooth decay is caused by the factors I listed above and cannot be attributed to a poorly fitting restoration.
Fracture of restorations tends to occur more frequently in some patients than others and this risk factors are bite forces and tooth clenching and grinding habits. Certainly the early failure of a restoration due to fracture in a patient without these factors and without a history of breaking restorations, can indicated some problem with manufacture or design of the restoration. In these situations, I typically remake the restoration with out charging and the problem is usually solved.
Forcing dentists to provide a five year warranty on crowns and bridges will only drive up the cost of care for all patients to cover the cost of replacement in patients with risk factors that cause higher failure rates.
Here are the links to the two paper I mentioned earlier.
http://www.ncbi.nlm.nih.gov/pubmed/18341238