It wouldn't neccesarily be a dead city, or even suffer from the loss. With a pair of lochs, the old riverbed could easily be converted into a shipping canal. As I recall, the Miss is fairly flat south of Baton Rouge, so it should require minimal dredging at the upper end of the channel to keep the whole thing filled with water. One loch at sealevel, and a second where the new shipping channel meets the river, are all that would be required.
Bingo...this is something I have argued for years, though admittedly I have failed to study the feasibility in detail. Still, the subsidence is natural, and will continue to occur...so we must be prepared for potential consequences to any development we allow to remain in the lower reaches.