Without the Civil War Slavery may may have gone away anyway, it may not have. The point is its not possible to say one way or another for sure and assuming it would have is not logical.
Well, I suppose we will disagree about this. I think, it is very logical given the fact that slavery WAS abolished in the rest of the civilized world without most of them having to go through a civil war. Many countries had already abolished it BEFORE our Civil War. Many more did so afterward. It was change in culture sweeping the world... long before, and long after the US Civil War.