wouldn’t argue your point. Could easily see Southern states with slavery into the 20th century, if the war had ended differently.
Given their investment, both in chattel property and in ideology, combined with their petulance and pride, I totally agree with you - they would have hung on to the bitter end (just like they did).
I used to despise the confederate rebels for attempting to destroy the United States. However, after more reading and thought I came to the same conclusion as you did, the evil of slavery would have most likely continued into the 20th century if not for the civil war.
If the southern states had not rebelled they would have had enough votes in congress to block most, if not all, of the republicans anti-slavery efforts.
Can see slavery in the USA till way past the 20th century if the South had remained in the Union. Longer if Lincoln had passed the Corwin amendment.
So if slavery is the lynchpin of your justification, how do you deal with the issue in what would have been the probable future of continuing Union slavery?