Is a post and core a good idea? I am looking for something that will last a very very long time. (photo)

A post and core is used to rebuild the structure that you have lost of the tooth so that you can be restored with a bridge. Unfortunately, as an oral surgeon, I can tell you that because of the way crown and bridges are made, it's an imperfect process at some point you will lose the cement underneath the crowns and end up developing decay under the crowns. My new philosophy with patients that need root canals is to offer my patients the possibility of restoring the area with implants which have a much better prognosis than a root canal, or even a dental crown.

If you choose not to do the post and core the other option is to remove both teeth and place implants. Your dentist can not guarantee that the post will last the lifetime of the bridge but your other option of removing both teeth is rather aggressive at this point. 

Good picture!Your dentist has a bridge he is ready to seat, and now part of the tooth preparation is missing.  A post and core should work fine as long as the margins of the bridge seal against natural tooth surfaces on both teeth. The question is, will he be doing a custom lab fabricated post and core (another impression and more wait time) or a prefabricated post and a build up around the post?The lifespan of the bridge is not real long with the forces of biting and chewing already having broken the tooth preparation. So, save your money while you have the bridge, and plan on an implant. That, should last you "a very, very long time".

Having no x-ray and not being able to see clinically how much tooth structure remains it is impossible to give a definitive answer. However studies will show that dental implants last longer than a root canal tooth with a post and core under a bridge.  You may want to consider a second opinion before completing the work.