Severing the Union posed no threat to government by, for, and of the people.
Even if Lincoln truly expected it to perish in the Southern States after succession, there was no reason to suppose it would not endure in the North.
Could be Lincolbn was a nuts as John Brown, but more likely he was just a practitioner of the “Big Lie” technique.
I posit that it did in Lincoln's mind. Fracturing the Union into smaller countries would eventually lead to continued divisions and destroy the American experiment. Kind of what we are facing now.
Lincoln was not nuts. He was quite brilliant and methodical, and I believe he was quite right about the threat the Southern Confederacy posed to the existing Union.
If the South had been allowed to remain independent, it would have eventually captured, through economic interest, most of the states in the existing Union. The Confederacy eventually had 11 states, and if a state of war hadn't developed, it would have eventually had the Union slave states, otherwise known as the "border states" join it.
The European money and goods that would have flooded the Southern markets would have eventually found their way to the Midwest and the further western territories by way of the Mississippi river and to other northern states along the porous border over which it would be impossible to guard against smuggling.
Over time, the Confederacy would have become more powerful than the Northern Union, and it was precisely because Lincoln and his New York backers could see this that he needed a war to stop it from happening.
The New York/Washington DC axis has ever since protected it's power jealously. The media systems in New York and Washington exists to maintain that power by controlling what people hear on the public airwaves.