If those cities were economically viable, then it wouldn't matter if we "abandoned" them (i.e., didn't give them federal taxpayer handouts, which is what you seem to be advocating). They'd have sprung up again anyway. The same goes for New Orleans. If it's economically viable to have a city in that spot, then one will grow back up again there.
well with the proper hurricane/flood protection, at that level it shouldn't matter. it was only protected against cat 3...