Splitting Lincolnian Hairs

I’ve had the opportunity recently to do some extra-careful thinking about Lincoln, the founding, and the Union. I’m pretty sure I’ve decided that many nettlesome and momentous theoretical issues came to a head in one relatively small practical question. What degree of peril did the secession of the Deep South expose to the existence of the Union that was embodied in the United States which remained? (Followup question: what degree of risk should have been tolerated in appraising that degree of peril? So, even if the existential peril was serious but unlikely, it might still be worth fighting against.)

It seems plausible that the peril grew by a very significant amount once the Upper South left the Union. But the Upper South, of course, only left the Union because the alternative was to invade the Deep South and/or become a bloody, Kansas-like battleground. So isn’t it possible to agree across the board with Lincoln’s most ardent defenders (practical and theoretical), yet decide that the civil war needn’t have happened — if it was unreasonable or unjustifiable to interpret the departure of only the Deep South as an existential threat to the Union? I’m still pondering this important question.