Gee, I'm sorry to have just seen this discussion. As Little Hawk says, it is fun to speculate on alternate histories and what ifs.

I am not so sure that the union could have remained together peaceably short of maintaining slavery, or even allowing it to expand. Whenever new states were being considered, the South fought tooth and nail to allow them, or even require them, to allow slavery, hence the Missouri Compromise and other acts.

Remember, slaves were considered 3/5 of a person for purposes of the state's representation in the House of Representatives, but the slaves had no say themselves. That was a powerful political tool for the slave-holding states. If new states were not slave states, this would dilute southern states' power in the house.

By the election of 1860, it was pretty well known that a Republican/Lincoln victory would cause a rupture of the country; South Carolina seceded, attacking Fort Sumter before Lincoln's oath of office was barely out of his mouth.

I suppose Lincoln could have allowed the secession states to leave with his blessing, but that was probably not ever a starter. Countries don't just let themselves be nibbled out of business. (Please don't bring up the Soviet Union and the Eastern bloc break-up; those countries were held by force in Russia.)

How much longer might slavery have continued in the south? Who knows. Might the slaves have successfully revolted as in Haiti and elsewhere? I don't know. But clearly it would have gone on for decades, as the slave owners were in no hurry to end that vile institution.