As to the call at the end of the [Mike Church Show] today, I appreciated the humility of the answer you gave. Slavery was on its way out in the western world, but as you said, it is all speculation as to how something might have turned out under circumstances that never took place.
Right now I am in the middle of reading "The Lincoln War Crimes Trial: A History Lesson"  on Lew Rockwell's web site. I look forward to pointing people to that article, but that is aside from the point.
As far as "what would have happened" I sometimes wonder how history would have been written had Adolf Hitler been more successful. Hopefully not in my opinion and understanding alone, many people were very concerned about the violence against many groups of people, but with the piles of propaganda produced by the state, the passive supporters of the Nazi regime accepted the belief that the parties ambitions had a moral imperative.
Similar to global warming, there was a belief that (rather than carbon dioxide) bad genetics was going to wipe out the human race if drastic measures were not taken to stop this downward spiral. If you believed this argument, what could possibly be more important than working together to combat this problem eventually by any means possible.
If people revered today the legacy of Adolf Hitler with the degree of revisionist history taught in our public schools and espoused by neo-cons about "Honest Abe", I expect that instead of the question "How would slavery have ended?", you would get callers asking how else the human race would have been saved.
And sadly [Mike Church] would have to give exactly the same answer: "I can't speculate about a history based on circumstances that never took place".
If I could speculate, slavery was on its way out as it was losing economic viability on its own as in the rest of the Western world. A spontaneous order would have been an economically strong south without slavery, and without the slaughter of a Civil War. The institution of slavery would have faded into the past and regarded with a disdain between the treatment of American Indians and Chinese railroad workers.