"Total nonsense. Slavery was dying. The advent of machinery was speeding up the obsolescence."
This is part of the reason why slavery was not that important a reason for secession. The northern example of paying people hourly wages without benefits (Industrial Revolution) proved cheaper than raising workers from birth and supporting them past their useful working life.
Second, only five percent of Americans owned more than slaves and only another two percent owned more than three.
Slavery worked against the Southern whites economically but they fought with everything they had for secession. Why? Because they hated northern attitudes and their states' loss of power in Washington. Northerners fought not to free the slaves but because they were conscripted to preserve the Union.