No can do. Can't get grain to market. This will starve millions in areas of world. Water is the only practical way to move this much weight to markets in other continents.

. So, over a century-long period they can't build new port facilities? Add rail "bridges" (in the sense that the Panama rail line, before they built the canal, was a bridge) to move grain over the now-dewatered sections? Because, you know, I'm
pretty sure most of the physical facilities of the Port of New Orleans are less than a century old (even if the port itself is, of course, much older).
The whole
point of managing the change slowly and gradually is so that problems like this can be
dealt with instead of continually building up the
existing facilities so that
when the Mississippi changes course you have a crisis like half the world starving because major transshipment points for US grain are now useless. And I really, really, really doubt that shifting 0.06% of the Mississippi's flow per month (or 0.002% per day) is going to cause ships to be unable to navigate the Mississippi all of a sudden. (And this is assuming that a "diverted" Mississippi would have a 100% flow down the Atchafalaya River, rather than, say, simply reversing the present 70/30 split so that the minority flows down the present channel. If you did
that, you would be diverting 0.001% per day). Given a high-end mean annual flow for the Mississippi, about 20,000 m^3/s (according to Wikipedia), this would mean changing the amount of water moved by the main channel in a day by about 17-18000 cubic meters, on average. This sounds like a lot of water, but it's about the same volume as a cube 25 meters on a side. It's less than a second of the river's discharge flow. It really isn't significant to navigation.
The corp has more of a "conquer nature" mentality than "work with nature".
Well yeah, of course.
Then you can get to the details of what it means. Manageable is not cheap. So once you decide to do it, the real go point is appropriating a huge amount of money (I would guess 200 billion USD) to start the five year process.
Well no, it's not cheap. What's also not cheap is fighting nature the way the corps does. What's also not cheap is having tens of billions of dollars in investments go down the drain because you chose to fight a battle you were inevitably going to lose rather than start a long-term process to mitigate it, like I proposed.
Given that I was talking about a 100-year process, too, it wouldn't cost nearly that much. Most of the actual funding would come in
routine spending, where you would build or repair existing infrastructure to fit the planned limits or private industry would build new facilities in new locations rather than reconstructing old facilities which will become useless. That's the whole point of doing it gradually, you don't
need to spend hundreds of billions or trillions of dollars to make the changes because your
intent to do so spurs, well, markets to do a lot of that spending instead, or local government.
Morgan city is not large, but it is critical. It is where much of what supports the off shore industry is located.
Fair enough, but there are other ports that are major offshore support bases, like the Houston/Galveston/Texas City area, or the Beaumont area. And, again, this is supposed to be a gradual process, so all this investment won't be irrelevant at once. Instead, what would happen would be the relevant corporations choosing to invest in facilities elsewhere while they work out their equipment in Morgan City. Or just in different parts of Morgan City, where the enlarged flow won't actually have a negative effect.
You would have to dredge down about 120 miles of river by 7 feet. This is 7 feet by 1500 feet wide by 120 miles. Lot of dirt. Build pipe line to bring in water to city of 1 million (NOLA). Not sure where this comes from, put probably water intake north of Baton Rouge. Then some dry summer (lower water time), you would need to start diverting the water. Few % at a time.
You know, if you actually read what I wrote, it would help. I was envisioning a much, much longer-term process, not merely a controlled version of the "flood breaches Old River Control Structure" scenario. Of course if you do that you have to spend a lot of money and go to a lot of trouble--that's why you
don't do that! So you dredge out that area by slightly increasing the amount dredged routinely, so that over time all that extra volume will be taken out. You don't spend billions of dollars on relocating oil refineries and chemical plants and so on, you tell ExxonMobil and BP and Total and all them that, well, in fifty years or so it's going to be real hard to reach those plants, so they might want to think about spending on building new ones rather than upgrading existing ones. You don't need to make special arrangements for shipping grain for one harvest, or at worst you need a relatively short rail bridge from the branch point to the new terminals at Morgan City or thereabouts. Nice, gradual, not horribly expensive, not the result of a disaster.