Being that the Superdome is now an official part of the U.S. Register of Historical Places, it will very likely never be torn down. I have a very hard time seeing a new stadium ever get built in New Orleans. They will just continue refurbishing the dome.
The Superdome, though it doesn't have nearly the same # of years on it, has essentially become akin to Lambeau Field and Soldier Field in terms of historical significance IMO, and neither of those stadiums are ever going away either.