In the 1700s, Thomas Jefferson largely wanted to uphold slavery because he feared what would happen if it were abolished: "But, as it is, we have the wolf by the ear, and we can neither hold him, nor safely let him go." The founding fathers obviously kicked the can down the road, but even many of the slaveowners would have hoped that slavery might fade away with time.
By the mid-1800s, John Calhoun's started going on about how slavery is a "positive good".
So... what happened? Did the nature of American slavery actually change in some tangible way? Is this just a matter of building inertia, where half the justification is that slavery has existed for as long as anyone can remember? Or did the surge in abolitionist sentiment cause slaveowners to dig in?
It's kind of an interesting progression to be completely honest. It shows how a single generation could change the outlook of a very controversial topic in just a short period of time (in perspective of everything else.)
This isn't my specialty (military history), but my professor and I spoke long enough about things like this that I can field a good answer. Short and simple, it's a mix of all the things you listed.
So like I said, this was more of a generational thing. The 18th century colonists looked at slavery as a necessary evil. The New World wasn't going to start itself and it needed workers, voluntary or involuntary. A lot of people recognized that and slavery wasn't really talked about amongst the populous, but it existed. In fact, indentured servitude, where those from the Old World could sign their lives for free passage to the New World to work, was quite popular at the time. Again, someone needed to do the work, by any means necessary. This is where the "positive good" that John Calhoun said stems from. As families began to prosper in the New World, they also began to grow, with more and more children being born in the Americas. Many in the South were born into families with slaves. As with any type of generation, the ideals and norms that they grow up with become their total view, and since the drive and speed of information at the time was quite slow, no one really got on the abolitionist train in the South. They were just too far away. Come the 18th century, when the next generation is now adults, they grew up with slaves. That's their view and that's how their parents made a living. It's how they are going to make a living. So when the surge in abolitionism did finally happen when information could travel faster, the slaveowners held their ground and refused to give it up, since that is how they made their livelihoods happen.
Hope this helps!
I'm not sure I agree with your premise: as early as I've been able to find records describing the enslavement of Africans, there are those who talk about its positives. Here we have the Sewall-Saffin debate, with Saffin basically saying that there is no better way to treat Africans than to enslave them.
To which we answer, That it is no Evil thing to bring them out of their own Heathenish Country, where they may have the Knowledge of the True God, be Converted and Eternally saved.
Hell, positive justifications of slavery date back to, in my reckoning, Aristotle in Politics
those who are as different [from other men] as the soul from the body or man from beast—and they are in this state if their work is the use of the body, and if this is the best that can come from them—are slaves by nature. For them it is better to be ruled in accordance with this sort of rule, if such is the case for the other things mentioned.