On thing about these slavery arguments:
One assumes that slavery has to look like it did in Mississippi in 1830s, or it's not slavery. Sharecropping was good enough for the land owners up until the 1930s, and I don't think that type of arangement couldn't be altered financially and socially to the point that Sharecropper couldn't = slave.
(No one says that a slave owner has to provide food or clothing to his slaves, instead of giving the slave a stippend to provide his own. Hell, often the masters didn't provide food, clothing, ect to slaves. Just gave them some time off to provide their own, and a little piece of land on which to grow their own food. The MS 1830s view of slavery really colors and warps the preception of what slavery necessarily has to mean)
Not to mention the possibility of industrial slaves. Sending a certain number of slaves to work in an steel factory, coal mine, anything thats labor intensive while not being particularly intellectually intensive is quite possible. (West Virginia supposedly had a number blacks in it back in the boombing days of coal mining)
Paying these slaves isn't out of the question either, especially if they have to spend their money at the owner's store. You just have the benefit of them never being able to legally strike or switch jobs for a better deal.
I could see slavery lasting well into the 20th century in the C.S.A. No, not in the since of a bunch of black people picking cotton in the fields, being whipped on their backs by a driver. But hell, a slave could work at McDonalds as well as a high-school kid or new immigrant. And when there's no longer a need for him, or you're downsizing, there's no reason he couldn't be put on a national slave stock market with other slaves, and bought by an expanding pizza-hut at bargin prices
One of the real limits of slavery is that it's not as easy to hire / fire slaves as it is free labor. But that's not a restriction smart men couldn't over come. Hell, people came up with the Credit Union to get around the exploitations of Banks. A Slave Holder's Union, or communally owned slaves, could get around the...I need 100 slaves in may, but only 40 in September....problem. Someone needs 100 September, 40 in may. And if they're within 100 miles of you...
Slavery could survive much longer if:
1) Slaves would probably be allowed to own property. Doesn't mean their property rights couldn't be abriged or ignored, but there would probably be some provision for it.
2) Slaves Codes would probably develop, allowing slaves some minimal amounts of rights.
3) Divide and conquer was implemented efficiently, creating a class of 'super slaves or free blacks' with great incentive to keep the lower slaves or regular slaves in check. Mullato families would be perfect for this position. If the Lousiana ethic could be exported to other Anglo-States.
Anyways: There's no reason to believe that the Southernor's first response to the declining profitability and acceptability of 1830s Mississippi Delta style slavery would be "Just chuck the whole thing" rather than, "Alright, how can we make this work?" Sure, slavery would become more complex. But then again, who'd believe the modern U.S. tax Code? The Modern C.S.A. 'Slave Code' might be just and ponderous, and full of loopholes, but just as real.