The irony of secession is that without it and the Civil War, slavery would have continued much longer in the United States. How would it have ever been ended? Even in Delaware, with 2000 slaves (outnumbered by the free persons of color), the slaveholders refused to agree to gradual emancipation during the War.
The mechanization of agriculture by 1900 would have been a death blow to slavery. The worst blow of all to the South after the war was the death of Lincoln. He likely could have controlled the radicals in Congress and favored sending free blacks back to Africa. He also said repeatedly that the South had only to meet minimum requirements to return to the union. Having said all of that, slavery was the most catastrophic mistake this country ever made and will likely destroy us in the end. Bringing Africans to these shores was a horrible mistake in hindsight. Likely a fatal mistake.