Can you provide a single pre-rebellion quote from a single Southern leader, military or civilian, who believed slavery was destined for an early end?
There didn't need to be one as it would have been an economical fact. I live in a rural area. Mule teams plowed the fields for centuries where I live. I do not own a mule. I use a tractor. A mule is a liability. I would have to feed it. Give it shelter. Tend to it's health as the loss of it would be a severe hardship. The tractor I can park until needed and it's needs are few. Apply that concept to slavery. As sure as it was dying in the north and a liability to most all persons yes it was dying as well in the south. It would not have lasted past 1890-1900.
Who cares what they believe?
Economic realities tend to drive these things and they are best determined in retrospect.