Slavery was an abomination! However, states rights were the issue. I do agree that if the south had won, it would have been very bleak for the slaves and the South in the long term, but eventually, the entire system of slavery and their agricultural economy would break down .
States rights and tariffs were the issue. Ironic considering that the modern GOP has flip-flopped and is now fanatically pro-"free trade".
An independent confederacy would have realized, just as the early independent USA realized, that you can't build a modern economy entirely on an agrarian base. You need industry, and it's easier to develop industry if your own native producers aren't driven out of business or never come to exist in the first place because your country is flooded with cheap foreign goods. In other words, "protectionism" works; it worked for Britain, it worked for the USA and Germany, it worked for Japan, it is working for China. "Free trade" is the mantra of the imperialists who want places to dump their goods and sources of cheap raw material (which is all an independent South would have been to Britain unless it changed its ways).
An independent South would have either industrialized and set up its own tariff barrier, which would have made slavery economically unproductive resulting in de facto emancipation sooner or later, ie, the South would have simply recreated the "American System" of Clay and Lincoln, ironically, or it would have stubbornly held to its agrarian bias and gone the way of South American countries like Argentina, or Brasil, which, also ironically, emancipated their own slaves in the 1880's because even agriculture was becoming industrialized and slavery wasn't making much economic sense anymore. In other words, slavery was going away no matter what, and the South would have recognized this sooner or later. What the South could not abide was being told what to do by the North, which was the real issue, not slavery: States Rights, federalism, etc.
IMO it would have been much better to let the South realize on its own terms the non-viability of a slave economy, rather than fighting a war and setting the foundations for federal tyranny to force them to be "righteous" like the North was supposed to be. But again, it wasn't really about slavery; it was about who was calling the shots, who paid the tariffs, who got to spend the federal money, etc.