[BroJoeK]

central_va: "The existence of USA was not threatened by the Civil War.
The union between the north and the south was at stake but not the USA’s existence."

In fact, as DiogenesLamp happily points out, had Confederates won they would have strong claims to states like Illinois, Indiana & Iowa whose exports shipped down the Mississippi to New Orleans.
Washington. DC, itself could become the Confederate capital, surrounded by Confederate states.
The rump US capital would once again flee north, to Philadelphia or New York.
So North America would split up into its several constituent parts and the United States would be no more:

[DiogenesLamp]

In fact, as DiogenesLamp happily points out, had Confederates won they would have strong claims to states like Illinois, Indiana & Iowa whose exports shipped down the Mississippi to New Orleans. Washington. DC, itself could become the Confederate capital, surrounded by Confederate states. The rump US capital would once again flee north, to Philadelphia or New York. So North America would split up into its several constituent parts and the United States would be no more:

Not exactly my argument, but not that far off. The South would have come to dominate the Western and border states, and those would have eventually come into it's political orbit, but none of this would have happened immediately. It would have taken decades.

Lincoln knew the threat that the South posed, and that's why he knew he had to keep places like Kentucky and Missouri from joining the South. Had those gone, others would have eventually gone too.