Do I read your correctly? Are you saying it is defensible? Even the most die-hard slavery advocates of the anti-bellem period didn't attempt a straightforward defense of slavery. They only justified it based on the "inferiority" of Negroes who were "born for servitude" to white men saying it was their "duty" to Christianize and civilize them through slavery. Even considering the dominant racial attitudes of that period, I doubt that very many of the slave advocates really believed that justification in their hearts.
We have to keep in mind that there hasn't been any time in recorded human history without slavery somewhere on this planet, including now. There's got to be something good about this system, the only other thing that has permeated human history so thuroughly is religion. Now we might not like the style of it, but we also get a very different view of slavery. Our knowledge is based almost exclusively on southern race based no way out slavery, which was an aberation in the realm of slavery. We also have an unrealistically brutal view of slavery thanks to things like Roots, a view that is not held up by the historical record.
Reduced down to it's basic form all slavery is, all it's ever been, is an exchange of labor for room and board. At some points in history that relationship was entered into involuntarily and sometimes voluntarily. But it's really not all that different from the laborer relationship we see defined without the word slavery. We all work for room and board, but non-slaves get to pick their room and board and slaves have that decision made for them.