Serious question: Without Southern secession, when would slavery have been abolished? It wouldn't have been 1865. The treason helped bring it to a quicker end.
Revisionist/apologist history in the 1950s/60s said, of course the South was moving to abolish slavery, it was declining of its own weight, slavery was suited to agrarian economies and would not have adapted to industrial production, etc etc.
We were assigned historian McKinley Kantor's "If the South Had Won The Civil War" (1960) in 7th-8th grade history class, and basically its premise was everyone would have gone on "happily ever after" in the three nations -- the USA, CSA and Texas Republic -- and that slavery would have been abolished in the CSA and Texas in the 1880s.
It was also posited that the CSA would have built quite a colonial empire in the Caribbean, including inroads in South America, and (portrayed in a positive light) would have established strong ties with South Africa. The three "American nations" were described as buddy-buddy comrades in both world wars. The main major geopolitical aftereffect for the 1960s reading audience was that the USA, wobbling after the humiliation of the CSA's successful secession and economically staggered by the war, had been unable to pull off the purchase of Alaska from the Russians, and so in 1960s, "Soviet Alaska" constituted a huge threat.
Of course, in reality, in any of the Southern states in 1860, anybody who suggested the abolition of slavery was at the very least, very quickly and vehemently shouted down -- or much worse. I see no likelihood of any movement against slavery certainly within a couple generations, and probably not within 100 years. Certainly Hitler and Stalin demonstrated that slave labor could be used in industry.