No one is defending slavery. My only point is that to say the slaves were 50% of the assets of the South is incredulous. Now, if your recent discovery is true, you have dropped the market value of the slaves even further as there marginal rate of substitution has been marginalized. In other words, if slavery was not profitable, the value of slaves drops like a rock.
I am unclear why you think the article posted shows slavery and value of slaves to be lower than expected. The article actually argues against those who claim slavery was unprofitable, demonstrating that it was considerably more profitable than other potential ways of investing capital.
ROI varied from around 10% to 16% from 1830 to 1860, and tended to increase towards the end of this period. Most people today would consider that a damn good return, especially in absence of capital gains tax.
IOW, value of slaves was increasing, quite dramatically, over this period.