Cotton was king because of its profitablety.
Planters bring ruined would drive down the price of slaves as slaves are sold at auction to pay creditors.
Slaves not getting as much on the market would hurt the banks trying to recover their losses.
A possible Southern economic collapse is possible as the Southern economy isn't as diversified as it would be in later years.
A Southern economic collapse might have
weakened Northern opposition to the spread of slavery, as so much of that was driven by the fact that the South had largely controlled the Federal government before the first Republican President (Lincoln) was elected. It might have driven slavery into the Southwest on a large scale, leading to a California dominated by San Joaquin Valley growers with their own black slaves, or extremely low-cost black labor performing all farm duties in those Southwestern states if slavery were not legal there. Alternatively or additionally,
the “Big Five” in Hawaii might have turned to unneeded black slaves as sugar labor rather than to Asians, as they would certainly have been as cheap if available abundantly from the Southwest.
A major black migration to the Southwest and/or Hawaii would have precluded the Great Migration to Northeastern and Midwestern cities, as blacks would have been further removed physically from those areas than they were in the South. It would also have reduced Mexican and Philippine immigration and possibly meant earlier Philippine independence. It is, however, also possible that with no Fourteenth or Fifteenth Amendments – as a Southern collapse would have prevented the war that produced them – that exploitation and racism would have been as or more severe at the height of “scientific racism” in the early twentieth century. Also, differences in culture between the South and Southwest make it possible that struggles for black political rights would have been much more violent if there was not the social support from churches provided in the South.