I think that the mechanization of cotton picking and processing was making the need for human labor obsolete. Very few of the Southerners owned slaves. To your question about what would have happened if the Southern states had successfully split off, I think it depends on how long it took to recover from the devastation and not suffering at the hands of the Reconstruction carpetbaggers who made matters even worse?
Here is an excellent summary of the issues from someone who was living through it:
http://freenorthcarolina.blogspot.com/2014/05/if-i-were-southerner-as-i-am-northerner.html
Uhh, no. There was little if any mechanization of cotton picking till well into the 20th century. And it was the mechanization of cotton processing via the cotton gin that made slavery wildly profitable and ensured conflict would result.
Very few of the Southerners owned slaves.
Not exactly. About 1/3 of all southern families owned slaves. Percentage of southern families that owned slaves in 1860 varied from 12% to 23% on the Border to 46% in SC and 49% in MS.
Since by definition those families were the more prosperous and therefore influential, they dominated the culture and politics of their states.
http://civilwarcauses.org/stat.htm
Mechanization of cotton harvesting didn’t come until the 1940s. Is that when slavery would have died out?