Whether or not Lincoln's unconstitutional actions to "save the Union" were justified, the reality is that if he had never been elected, there would have been no Southern secession, and no Civil War. Slavery in the South would have continued somewhat longer, but within a few decades of the historical Emancipation would no doubt have been abolished, as it was in every civilized nation. American Negroes may well have been better off if there had been no Reconstruction, and the Southern White KKK reaction. Even a victorious CSA would no doubt have eventually abolished slavery.
Interestingly in the early Republic even slave owners acknowledged the evil of slavery, views hardened in the face of Abolitionist agitation. See Spreading the News.
Slavery in the South would have continued somewhat longer, but within a few decades of the historical Emancipation would no doubt have been abolished, as it was in every civilized nation. There was not a single case were slavery died on it's own. In every instance it was ended through the actions of the government and in the face of strong opposition from the slave owners themselves. So if the south was willing to go to war to protect slavery from government action in 1860 then how long do you think it would have taken for such action to have been acceptable to the southern population?