All excellent points. I was at the New Orleans Archives museum a few years ago and the docent who was showing us documents in the collection said that to the Southern slave owners, it was as if the federal government were suddenly taking away their farm equipment. Some of them were into African inferiority, some of them not, but all of them relied upon this - as you say, cost-inefficient, outdated - labor. However, they probably would have been amenable to solutions.
I’ve always blamed Charleston for the Civil War, not only because they fired on Fort Sumter, but because they had a whole strange theory of slavery and southern life worked out. But keep in mind that it was South Carolinians who destroyed the Spanish Franciscan Indian missions In Florida and either killed or enslaved the Indians, and also particularly wished to get rid of the Spanish presence because slaves who made it to Florida were free.
South Carolina was very committed to slavery ideologically even after England ended it. It was also the scene of the Stono Rebellion because their treatment of the slaves was so bad. When they put down the rebellion, they punished some of the leaders by burning them to death before an audience in downtown Charleston.
And of course the first Democratic Convention of 1860 was held in Charleston, where they effectively managed to get rid of somebody who would have been more compromise-oriented on the slavery question. South Carolinians, much more than any other single group in the South, regarded slavery as the be-all and end-all of Southern existence, and they dragged the rest of the South along with them.
There was an article some years ago about the Stono Rebellion which argued that the slaves involved were Catholics (from the Kingdom of Kongo, where the king had converted to Catholicism) who were trying to get to Spanish Florida hoping that the Spanish would recognize them as free. The Portuguese and Spanish believed it was OK to enslave pagans but not Christians.