Let me clear your moral qualms. Had the South remained in the Union, Slavery would have persisted into the 20th century. Lincoln repeatedly tried to bribe the south into staying by offering them assurances that slavery would be protected.
Beyond that, the numbers just don't work. To ban slavery would have required a constitutional amendment, and that would require a 3/4ths vote of all the states. With the 11 states that became the confederacy, it would require a Union of 44 states to override their opposition. (Not possible until 1896.)
If you add the five Union slave states to the 11 confederate states, it would require a Union of 64 states to override them, and we still do not have 66 states in the Union.
In other words, the Union would have kept slavery because there was no possible way of winning the legal battle to abolish it.
Therefore the war really wasn't about slavery. What it was about was the potential that the money pile coming into New York would end up in Southern ports.
There was 200 million dollars per year at stake immediately, and with the capital moving to Southern States, there was even greater financial threat to New York (and the Northeaster power barons who still run the United States today) in the possibility that the South would not only build competing factories with that money, but also that they would eventually provide the Western states with goods imported or manufactured in the South, and thereby cut off that source of money and power from the North East too.
The war was about money, and only about money. They created the propaganda that it was about "slavery" because they didn't want people to know the real motives for it. If they knew the real motives, the public would have been against it.