It seems slavery would have become increasingly anachronistic as technology progressed. Slavery might work in industries requiring repetitive labor such as cotton picking but would be poorly suited to high-tech jobs requiring high degrees of motivation and skills that were increasingly in demand even in the rural South.
The march of time would have seen slavery fall by the wayside with no other action taken. My guess is it could have lasted between 20 to 80 years longer, but that is probably it.
And if it lasted 80 years longer, it would be because people would be too embarrassed to admit they were wrong.