You should read what Washington wrote on the subject. I have read quite a lot of his writings on the issue of slavery, and in his time it was becoming hard to make enough money with slaves to make it worth their upkeep.
Of course as you said, the cotton gin changed the profitability equation, and so the demand increased dramatically.
But the next technological transformation would have had the opposite effect. Once it was noticed that machinery could do the work, there would have been a steady migration away from slavery and into machinery. This was inevitable. The only question is how long would it have taken?
They were experimenting with various farm machinery in the 1870s, but I think the first machine for harvesting Cotton didn't come about till the 1940s. Of course the plowing and the planting and such was being done by machinery around the 1900s, so that alone would have reduced a lot of the labor required.
Other factors are that the social pressure to end it would never have abated, and those wealthy people who would have the largest numbers of slaves would have had the means to pay for alternative technologies.
You point out that there was unimaginable Wealth in the South, and this is true, and if you've studied mankind for any length of time, you start to notice that "unimaginable wealth" is sort of a root cause for Liberalism. People cannot seem to help it. The second generations of "Unimaginable Wealth" always seem to support Liberal ideas, and I have little doubt that the same would have become true of the children of the Wealthy people in the South.
They would have eventually started putting "social welfare" ideas ahead of their own financial self interests. It has happened all over history, and I regard it as a constant in human behavior.
That's a fallacy. If you wait long enough, just about anything can happen, but "unimaginable wealth" didn't end apartheid until other factors came into play.
And of course, that wealth wasn't going to last. Cotton prices were bound to fall sooner or later, and with them, the wealth of the planters. That would leave Southern elites, struggling to hold on to power.
But also, it's surprising that your Marxist analysis leaves out another possibility -- with great wealth comes expansionist tendencies and the desire to conquer new territories abroad. Social welfare ideologies usually come into their own after geographic and economic expansion falters.
So it was that we acquired overseas colonies just before turning to progressive and social gospel ideas, and wealthy European countries devoted more energy to imperialism and militarism than to social policy -- up to the point where those wars and colonies became too costly.