Slavery was completely gone from its last holdouts in the West (Cuba and Brazil) by the 1880s. The idea that it would have somehow lasted in the CSA well into the 20th century even though it died out everywhere else in Europe, the Americas and the Europeans’ vast colonial empires is laughable. The reason it died out was economics - not some grand moral awakening. The laws of economics worked just as much in the South as they did everywhere else.
Slavery did not 'die out' in either Cuba or Brazil. It was ended by governmental order in both cases and in both cases over the objection of the slave holders themselves.
I understand your point and I find it highly plausible. However, an alternate explanation occurs to me.
It could be that slavery died out because the great powers created economic reasons for it to die out by throwing up obstacles or refusing to engage in trade with slave nations. Also the social pressure which may have been applied.
I think the truth is somewhat in between these two positions. I think it was economically declining anyway, and I think outside economic and social pressure contributed to other nations giving it up.
I think in these other nations it met my criteria for a declining economic trend meeting a rising Social trend, and these two trends met at those times in history and created a preference cascade that flipped the paradigm.