Katrina was "new territory" for New Orleans. They'd survived hurricanes before so folks always expected to survive them every time afterwards. All it takes is being wrong once.

It is possible that this time it is indeed different. There are a number of factors which add up to a different environment in which to recover which could inhibit or prevent the recovery.

As preparedness folks we ought to be willing to examine and prepare for the entire spectrum of possibilities. (i.e. Keep that axe sharp so that you can be first to the park to get your firewood :-(