The problem is this. Every patient and tooth is different. Some people grind their teeth, some people don't brush their teeth, some people don't floss, some people chew ice, some people have a high caries rate due to medical conditions, etc. The long term prognosis for any dental work is different from tooth to tooth. It's like doing a knee replacement on someone who is in perfect health and exercises regularly and doing the same knee replacement on an obese person with uncontrolled diabetes. Two completely different cases with different probabilities of success.  The key is patient education and expectations. They need to make informed decisions. Dentists must be beneficent and make case to case determinations on warranties considering all factors. This is in regards to all treatment...not just crowns.