I challenge you to find one southern leader who said before the war that slavery was dying. The fact is that both the number of slaves and the price of slaves was increasing. Hardly signs of a dying institution.
"There is evidence that suggests slavery was beginning to die out on its own. For example, the percentage of Southern whites who belonged to slaveholding families dropped by 5 percent from 1850-1860" (Robert Divine, T. H. Bren, George Fredrickson, and R. Hal Williams, America Past and Present, Fifth Edition, New York: Longman, 1999, p. 389). Nevins noted that "slavery was dying all around the edges of its domain" (The Emergence of Lincoln, Volume 2, p. 469).