Lincoln quite possibly could have ended the slavery and preserved the Union without war.
State conventions in both Virginia and North Carolina, which by far were the center of gravity and ended up supplying the lion’s share of men for Confederate forces, voted AGAINST secession the first time it was proposed.
It was only AFTER Lincoln demanded that these states supply a large number of soldiers for use in conquering the deep south states who wanted to leave the Union that Virginia and North Carolina decided to secede.
States like New York, New Jersey and Pennsylvania were all SLAVE states at one time. The outlawed slavery by law using sunset provisions.
This same sentiment to outlaw slavery by sunset provision saw increasing popularity in Virginia and North Carolina after the Revolution and the establishment of the Constitution.
It is quite possible that if Lincoln had held his horses for war and played his hand more intelligently, he could have managed to hold Virginia and North Carolina (Kentucky and Tennessee would have followed suit). In that case, the Union would have been much more powerful than the deep south states and after a period of time the deep south might have been cajoled back into the Union with sunset provisions to end slavery. All without war and the deaths of millions.
I like your theory but I think the average Southerner just plain hated being in the Union with Yankees. I think it is that simple. Southerners then, and now(at least some) do not like centralized authority. Especially unrestrained, irresponsible Central Authority.
And that was only AFTER the confederacy had chosen war to futher their aims.
This same sentiment to outlaw slavery by sunset provision saw increasing popularity in Virginia and North Carolina after the Revolution and the establishment of the Constitution.
Then why didn't they?