This is, unfortunately, an anachronistic argument.
In 1787 it was absolutely correct, one reason the Founders were always careful to avoid giving the institution any explicit recognition in the Constitution, instead always using euphemisms. Washington (highly honorably), Jefferson (much less honorably), Madison and pretty much all the prominent southern Founders explicitly denounced the institution as evil.
Unfortunately, by 1860 this attitude had been stood on its head, and slavery was believed throughout the South not to be "intrinsically wicked" and "damned to end," but rather to be a positive good the blessing of which they fully intended to spread throughout Latin America if they couldn't force it on the northern states.
If you have countervailing evidence, I'd love to see it, but in all the Declarations of Secession I've read, I don't recall a single statement about slavery being evil and to be brought to and end. On the contrary, they normally list the protection of the institution as the primary or only reason for their secession.
In fact, I doubt there is much discussion of slavery as an evil by prominent southerners during the whole decade of the '50s.
Lincoln, quite accurately, declared his attitude towards slavery to be that of the Founders, and that of the slaveocracy to be the perversion of their ideals. The secessionists quite openly proclaimed our founding ideal, "all men are created equal," to be false.
I believe that you are right on this point. I have not specifically researched this topic, but I have read many original ante bellum documents, especially wills and probate documents. I have a letter, written by a North Carolina ancestor in 1784 to his cousins in Ireland. He lamented the survival of slavery into the New Republic and predicted its end in due course. He also predicted that if the country could not craft a solution, a civil war would ensue. He was a slave holder and Revolutionary War officer. I have read many other contemporary accounts by the political and social elites debating how to end the institution.
But, by the 1850's, this talk had disappeared. I think in part this was just a hardened political stance in response to external attacks by the abolitionist movements in the North and in England. They took the stance that abolition was easy in England and the North where Industrialization provided the only economic engine required. The South, they contended, could not survive economically without slaves. It was a rationalization in my opinion, but it was one that was wildly held.
The other factor was the cotton gin. Before its invention, slave based plantation agriculture was a low country activity. In the Piedmont and Western states, the Scots Irish and German settlers held few slaves and operated small subsistence family farms. The cotton gin changed this. The upland farmers, with slave labor, could grow abundant crops of cotton and many of these poor Southerners became very wealthy men. Prosperity changes attitudes, and many who held moral objections to slavery adjusted their stance. Of course, when the vote to secede came, many of those upland dwellers voted against it, but they had become the minority.
BTW, all of my ancestors were Southerners, including some well known slave holders like Thomas Jefferson. With the exception of a very small group who left Tennessee and went to Illinois in protest of slavery, all of the rest supported slavery either explicitly, or implicitly through their service to the Confederate cause.
"I was much pleased the with President's message. His views of the systematic and progressive efforts of certain people at the North to interfere with and change the domestic institutions of the South are truthfully and faithfully expressed. The consequences of their plans and purposes are also clearly set forth. These people must be aware that their object is both unlawful and foreign to them and to their duty, and that this institution, for which they are irresponsible and non-accountable, can only be changed by them through the agency of a civil and servile war. There are few, I believe, in this enlightened age, who will not acknowledge that slavery as an institution is a moral and political evil. It is idle to expatiate on its disadvantages. I think it is a greater evil to the white than to the colored race. While my feelings are strongly enlisted in behalf of the latter, my sympathies are more deeply engaged for the former. The blacks are immeasurably better off here than in Africa, morally, physically, and socially. The painful discipline they are undergoing is necessary for their further instruction as a race, and will prepare them, I hope, for better things. How long their servitude may be necessary is known and ordered by a merciful Providence. Their emancipation will sooner result from the mild and melting influences of Christianity than from the storm and tempest of fiery controversy. This influence, though slow, is sure. The doctrines and miracles of our Savior have required nearly two thousand years to convert but a small portion of the human race, and even among Christian nations what gross errors still exist! While we see the course of the final abolition of human slavery is still onward, and give it the aid of our prayers, let us leave the progress as well as the results in the hands of Him who, chooses to work by slow influences, and with whom a thousand years are but as a single day. Although the abolitionist must know this, must know that he has neither the right not the power of operating, except by moral means; that to benefit the slave he must not excite angry feelings in the master; that, although he may not approve the mode by which Providence accomplishes its purpose, the results will be the same; and that the reason he gives for interference in matters he has no concern with, holds good for every kind of interference with our neighbor, -still, I fear he will persevere in his evil course. . . . Is it not strange that the descendants of those Pilgrim Fathers who crossed the Atlantic to preserve their own freedom have always proved the most intolerant of the spiritual liberty of others?"--Robert E. Lee letter dated December 27, 1856