> let the Mississippi divert itself away from New Orleans and Baton Rouge

This is perfectly workable, while I understand YMMV

> would be an unthinkable economic (and social) catastrophe.

Not only is it thinkable, it could be done responsibly, today. The ORCS will continue to fail (at irregular intervals), which will cause catastrophe as these events do. At some point, it will be decided that the system is no longer worth maintaining. Might as well rip the band-aid off by setting a specific date and start making a graduated change. If you didn't move out after Katrina[1], I don't think you can be convinced anyway.

[1] there was a 29% reduction in New Orleans population, post katrina