Tobacco had worn out alot of the soil, and was becoming very difficuly to grow.
Cotton did too though, which was one of the many, many things that prompted the Southerners to be so adamant about American expansion, settling of new territories, and when that wasn't an option, picking a fight with people who owned them. Basically, they needed more land (that wasn't exhausted by cotton or tobacco farming) and more territory with which to grow their extremely deleterious crops also it gave more representation and power to the South.
But in regards to the cotton gin...
It'll kill slavery as an institution to have a delayed cotton gin, someone's going to invent it eventually, you can't outright delay an invention when there's a massive demand for labor-saving inventions, if there isn't an American version, there will be a British or a French version for sure.
But aside from losing the title of inventor to one of the most important innovations the textile industry has ever seen, the Americans stand to benefit significantly if the wretched institution dies out early, while racial issues will still certainly be a problem, it's unlikely that they'll have the some radical violence on the part of the whites to try and preserve what they see as their system.
Economics wise, once slavery as a system crashes, there's going to be a ton of short term financial instability, a panic of the magnitude the young nation probably has never seen before, unless it is slowly phased out and simply replaced by wage labor the way a lot of moderate abolitionists wanted. Suddenly with a lot less division between the North and South the gap starts to close between the industrialized, affluent North and the old agrarian South as industry, banking, and some of the other nice benefits to being an industrial power start to work their way into the former South things are liable to get better? Will it be a peachy clean USA of freedom, justice, and equality? No of course not there's still going to be heaps of problems. Will it be better than the North/South divide that caused the Civil War in our time? Most likely.