The central premise of Time on the Cross - that slavery was economically profitable and unlikely "wither away", and this had some positive effect on the treatment of the slaves, seems quite plausible to me. (That said, I believe this is only true after the invention of the cotton gin).

The first half of the thesis is most assuredly true. It could be that if not for the invention of the cotton gin, slavery would not have been profitable in the cotton-growing regions of the US South, but slavery was extremely profitable and economically dynamic elsewhere, so I wouldn't be inclined to lay too much emphasis on the gin (except as a matter, possibly, of where slavery came to be located, as it did die out "naturally" in the areas where it was unprofitable.) However, it is also true that northern and/or metropolitan political leaders generally believed (however incorrectly) that free labor would generally be more efficient than slave, which to be fair it was in the industrial production processes that the abolishing regions had a comparative advantage in.

I am extremely skeptical of the second part of the thesis, because most everything I've seen indicates that slaves were worse off than black sharecroppers were worse off than southern whites were worse off than northern whites. But I haven't actually read Time on the Cross too closely.