Cotton exports were the primary US export from 1800 to 1930. You can see from the chart below that cotton was responsible for 57 percent of all US exports when the war broke out. You will not find any other export which had as big a footprint nor one which was the number one export for as long a period. Go ahead and try.
As I stated earlier, the slave population in the South was 650,000 at the time the Constitution was ratified. This is why the South was agreeable to the compromise in the Constitution which ordered the end of the importation of slaves by 1808.
However, between 1790 and 1808, the English and US textile industries exploded due to technological advances having nothing to do with the cotton gin. Everyone has heard of Eli Whitney, but few have heard of Samual Slater, "Father of the American Industrial revolution". This industrial advancement made textiles much cheaper, and thus greatly increased the demand for cotton. The demand for cotton drove the invention of the cotton gin, not the other way around.
The increased demand for cotton, in turn, required more slave labor.
So when the 1808 timeframe rolled around, the South began reneging on the Constitutional ban.
The slave population steadily and rapidly increased to the point that the slave population was 4 million in the South at the outbreak of the war.
Only a fool claims slavery was dying out.