> 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

> Not only is it thinkable, it could be done responsibly, today.

How?

You have a metro area of 1.3 million which is totally economically dependent on the river. A huge volume of freight moves through the delta daily. Billions of dollars of port infrastructure have been built in New Orleans and Baton Rouge.

Even disregarding the social cost -- which would be huge, given the high levels of poverty in New Orleans -- the pure economic cost would quickly rack into the tens or hundreds of billions. And tens of billions more would be required to make the new course navigable, rebuild port infrastructure, build new cities to serve the port, relocate workers from other parts of the country, etc.

And even then the river could just change its course again in two years or twenty or fifty.

Not to mention the time it would take to rebuild the industry that is between Baton Rouge and New Orleans. That combined with the fact that the major shipping route to deep water ports to export ag commodities (corn, wheat, soy) to the rest of the world would effectively be halted for probably a year would mean a multi-trillion dollar arrow to the knee for the US.