So how would it have ended, short of a war? Slaveholder had a lot of their capital sunk into their slaves, a field hand could sell for $1500 in 1860. How would the South have freed a third of their population without a fight? And if slavery's natural end was just around the corner, why did the Confederate constitution forbid any state from ending it on its own? So much for "State's Rights".
Economically, slavery was almost at an end, even though the planters didn't want to admit it. The Industrial Revolution would have forced an end to slave labor.