Slavery was already on the way out.
Land depletion and mechanization plus civilized opinion would have accomplished what it took Lincoln three years to declare.
Yes, in 1860 slavery was declining in such Border South states as Maryland, Delaware & Missouri, but the reason was because slavery was hugely profitable in the Cotton South and that prosperity drove prices for slaves so high they were no longer profitable in non-cotton states.
In 1860 there was no suggestion in the Deep South that slavery might not last forever.
Indeed, insuring that was the main reason given for declarations of secession, in late 1860 & early '61.