An institution doesn't have to create wealth for an entire country, in order to persist. It only has to create wealth for the rich and powerful.
The Civil War was absolutely about slavery. The South sure thought it was. The amount of ink it spilled on the subject of protecting that one particular right of a state was staggering. It was the one thing that apparently, they could compromise on. It's almost as if preserving the right to keep human beings like cattle was the whole bloody point of their secession.