The Straight Dope has threads on their forum (which at some point switched to Discourse) from 1999. So given that you consistently have access to your own data

The Straight Dope has threads on their forum (which at some point switched to Discourse) from 1999. So given that you consistently have access to your own data I don't see why a dedicated webmaster couldn't keep a forum active for a significantly longer time than you can trust Google to keep any product alive. Is it possible to export data from Google Groups? If not then longevity isn't the right question, they will not keep it active forever.