I took a course on the Civil War in college, taught by a grad student who opined that if the south had not seceded, slavery in the US would have continued into the 1920s. Any thoughts?
It depends on what you consider "continuing." The South with its "peculiar institution" was already far behind the North in terms of both development and economic prominence--and besides, many in the United States who weren't outright abolitionists were noticing that the United States was one of the only nations left with slavery.It seems more likely that it would have been killed off a bit earlier by progressive reformers... However, you also have to consider that after the failure of reconstruction states like Alabama became dependent upon prison industries in which the vast majority of inmates were African Americans arrested on dubious charges and worked nearly to death. Black men who became unemployed could be put on chain gangs.
I could see slavery continuing for a very long time, had it not come to military action. The financial interests in support of slavery, and the culture built around it, were unlikely to change out of the goodness of people's hearts.