Slavery was on its way out in the upper south for a variety of reasons.
Then explain why Virginia came near to banning slavery in the 1850s?
I believe Straha is mistaken on this one. Slavery was thriving further south and west, in big cotton states like Alabama and Mississippi, as well as Sugar plantations in Louisiana. In old tobacco states like the Carolinas, Virginia, and Maryland, slavery had been slowly dying out from the 1780s on. Cotton basically was what kept slavery alive, more-or-less.
An independent confederacy would have eventually banned slavery; however, it probably would not have happened in every state until well into the 20th century. Increased competition from cotton coming from India and Egypt would have made cotton less profitable, and I don't see the Confederacy gaining much in the way of slave industries.
