You’re exactly right. The Mississippi will be captured by the Atchafalaya, its just a question of time. The shorter distance means that the slope along the Atchafalaya is greater and the river wants a steeper slope to more efficiently carry all the water and sediment load. Keeping the river in its channel all these years has raised the bed of the river in its lower reaches and reduced the overall slope. The river doesn’t like it, and someday during a flood event, it will find a path to the Atchafalaya and that will become the new route to the Gulf. That’s how the Delta was built over many millennia.
Will they rename the Atchafalaya - Mississippi?