We already offer a similar warranty.  However, there are two issues that need to be addressed, since this only works if there is bilateral responsibility.

Patients must maintain a reasonable oral hygiene and examination protocol.

Trauma and accidents are not covered under the warrantee.

You can't have patients disappear for 5 years and then claim that your dental work failed, for whatever reason.  I've dealt with these over the years, and have had no problems with patients who I have been seeing on a reguiar basis, but have had problems with patients whose dental condition is degrading unsupervised, or have never returned for any followups, or what appears to be a trauma situation, etc.

Another option is to have a sliding scale of  "responsibility".  The expectation would be different at three months as opposed to 59 months after initial completion.  Most true failures will occur sooner rather than later.  Later failures tend to be neglect or trauma related.

Failure must also be defined.  Does failure of the underlying tooth structure consitute failure of the restoration?  Would a slight porcelain chip which doesn't compromise longevity or affect esthetics , and could be smoothed out, consitute failure of the restoration?  What about recession?   These types of issues must be dealt with beforehand.  It gets more involved in the regulatory areana if dealt with after the fact.