Minor issue. The major issue was tariffs on exported cotton and imported industrial equipment.
That would be news to the fire-eaters who drew up the ordinances of secession; they uniformly listed slavery as the main issue. I guess they were either too stupid to know the real cause were were lying. My guess is that they were lying because it was much better to declare slavery, not tariffs were the real cause because so many more people would get behind slavery and not so many behind tariffs.
The fact is that tariffs had been declining in the 15 years before the Civil War. The Morrill tariff, which was protectionist, had little chance of being passed — until the delegations from the seceded states withdrew. That of course, boosted the belief — aided by cotton interests in England — that the war was really about a tariff that would not have been passed without session and occurred after, not before, the Southern states withdrew.
Slavery would have died out on its own by the 1880s. It was, at best, a secondary issue.
There is no evidence to suggest this. The percentage of slaves as a share of the total population in the Southern states was increasing by 1860, not decreasing. If slavery would have "died on its own by the 1880s," why did the Southern plutocrats insist on gambling their entire society on something that would have passed in twenty years? Sure, they could have been deluded. But obviously didn't think so.
And to believe that the poor Southern planters were being economically strangled by the North is to ignore the fact that capital investment in slaves and cotton was more profitable than investment in the North and it just made good economic sense for them to put their money into land and slaves than into industrial development.