It’s an interesting exercise in counterfactual history. The human cost of the war was so large, that it makes us wonder whether allowing slavery to persist a little longer would have been an acceptable trade off.
The Union could have said to the South: OK, secede, but I’m not buying your cotton, and I’m not selling you anything either. Would the south had declared war to open markets to its products? Probably not: easier to trade with Europe, Latin America, etc.
My guess is that before long: (1) the Union would have been restored, and (2) slavery would have been abolished in the southern states.
Latest research puts the war’s death toll at 750,000. Too high a cost was paid by a country of 31 million.