Duh! How about, everyone was about expansion, and everyone was about taking his property, institutions, language, and politics with him when he moved into the Territories? You take a commonplace and make it "all about slavery", which is the McPhersonian Lie. It was "all about a lot of things"!
The whole nation participated in expansion. For the Southern planters to have accepted that they couldn't move to new plantations in the Territories would have meant acceptance that they were second-class citizens, from their point of view. It would be like saying that New Englanders couldn't travel west with their furniture and their Bibles -- in the law, there was no difference between a slaveholding Southerner's property, and a New Englander's. What the Northwest Ordinance did, was to impair Southerners' use of their property, without first modifying the legal status of the property -- bass-ackwards, in other words. The intention was to reserve United States Territories for free farmers, to the detriment of planters, and the exclusion of slavery was basically a gimmick without a constitutional basis. Job One for the free-soil and freeholder political factions was to impair slavery constitutionally (as in, the Thirteenth Amendment) in order to protect the anti-slavery laws. The failure to address first things first led to Dred Scott and the exposure of the entire Northern position on slavery to the rebuke of illegality and factional partisanship. Which it was.
Slavery distorted the economy by pouring investment capital into slaves.
"Distorted" is a value-loaded word, otherwise I agree with the general proposition, that it sucked up a lot of capital. On the other hand, what if all that capital had been mobilized and poured into that other 19th-century passion, which was labor-saving devices intended to reduce wages? If slavery hadn't vacuumed up all that capital, the deployment of that capital into union-breaking, wage-breaking devices might have turned the entire agricultural workforce, and the vast majority of the citizenry, into mendicant helots begging outside the wrought-iron gates of privilege. What if the Gilded Age had been even more intense than it was, and had started 40 years earlier? Don't just assume that "if things had been different, they'd be better." The English experience was a lot like what you posit, and for vast numbers of men and women who were driven off the land either into urban slums or into exile in the New World and Australasia, it was a bitter experience indeed -- and a lot of them died of it.
It distorted its peoples' social development by institutionalizing the fiction that one race was superior to and deserving to own the other.
The North eschewed slavery, but embraced racial theories enthusiastically enough to put down your assertion. Even Lincoln opined that blacks were not materially whites' equals: his contention was radical that they were morally whites' equals, and that they ought to be at least free, although he was less explicit about their having the franchise, because he had reservations about their intellectual maturity. That he thought Jefferson's words about "life, liberty, and the pursuit of happiness" ought to apply to blacks, put him at the cutting edge of what might pass for 19th-century liberalism, save for the white women who, generations before, and in defiance of custom, racial boundaries, and community opinion, had taken up with free black men and lived with them in (I should think usually) common-law marriage.
The rest of the community generally disagreed with Abolitionists who thought blacks ought to have the franchise.
Don't confuse the free-soilers' enthusiasm for shutting slavery out of the Territories, with any sort of liberal sentiment for giving ex-slaves the franchise, or anything that would pass as liberalism today. Their interest was entirely practical, and oriented toward the elimination of an important source of threatening economic competition.
Without slavery the development of the South would have been much more like that of the north. Its industries would have grown under the tariff, its labor force would have been free and able to develop as that of the North.
No, it wouldn't. The South had nothing like the mineral resources of the North -- just some coal seams and a little bit of iron ore, plus the Georgia gold fields, which were very modest. Its industries might have benefited from the Morrill Tariff, but it's arguable whether they'd have benefited enough, that they'd have been able to meet the economies of scale that Northern factories were achieving by scaling up and holding labor costs down.
After the cholera and yellowjack epidemics of the early 1800's, word got back to Europe about the unhealthful climate, and the South attracted far fewer Irish immigrants than the East Coast ports. So the South would have enjoyed less of a labor-cost advantage, and only transportation-cost advantages over Northern products. Too, capital was less extensively available in the South, since it tended to end up in New York banks, and there were fewer opportunities other than debt to persuade investors to keep their earnings at home. Except, as stipulated, putting more land into cotton.
If Hamiltonianism hadn't destroyed it during the War, it would have been some other modern industrial capitalist power since the system of slavery was in conflict with the entire Western world not just the Northern states.
Not necessarily true at all. The South was, until 1860, protected from foreign adventures by the United States Government.
The Ruling Class would have never given up its slaves, voluntarily.
The reflexive answer is yes, you're right -- but the planter class in northeastern Brazil, which answered closely in its structure and functioning to the planter class in the South, accepted manumission by imperial edict under the Braganca emperors without appealing to the sword. Whether they would have done so without the spectacle of the American Civil War is a question I don't have the answer to -- a Latin American historian would be needed to venture a useful opinion on whether, absent the ACW, the Brazilian planters would have acceded peacefully.
I've read comments on how the US was the only western country to need a bloody war to remove slavery. And on the Southern argument (noted by Mark Twain, I believe, which shows it was not a later argument) that it was a tariff war. So possibily the reason there wasn't a drive to labor-saving devices was that they were primarily of British manufacture and the tariff was prohibitive.