What do you mean by this? How can we lose the Mississippi? I'm curious. Illustrations would be nice if you have them. I grew up not far from the river.
The part that would be "lost" would be the segment from the "Old River Control Structure" to New Orleans.
The reason is that the Atchafalaya River is deeper and provides a MUCH shorter path to the Gulf of Mexico. The Mississippi has been "wanting" to cut a channel from its existing bed to the Atchafalaya for decades. The reason the "Old River Control Structure" was built was to prevent that.
In the high water of 1973, the Mississippi almost undercut the then-existing control structure, and to protect that structure was the reason the Morganza Floodway was opened that year.
Since that time, the original control structure has been re-inforced, and supplemented by the "Low Sill Structure"...a separate additional control structure to provide more bypass capacity.
If the Mississippi "does" manage to switch beds, salt water will intrude back up the now "slack flow" current channel, and communities (and industries) that take their fresh water supplies from the river would have to be drastically re-designed, or abandoned.
Search term: "Old River Control Structure" will take you to lots of info.
Here is an excellent illustrated history of the interface between the Red/Old/Atchafalaya River systems and the Mississippi River system.
And here's some further information on the Atchafalaya -- America's largest unknown river.
To sum up, the fact is from the Old River/Mississippi River junction, it is 175 miles to the Gulf via the Atchafalaya vs 325 miles via the current Mississippi channel.
Water seeks the shortest, thus steepest, route to sea level. If it weren't for the control structures, Morgan City would probably already be New N'awlins.