If the confederates were out of the Union, they had no obligation to follow the Missouri compromise. They would have gotten those western states above Missouri too.
So it appears that you share my opinion that the Confederacy did represent a serious existential threat to the United states.
Central_va wishes us to believe the Confederacy was no more a serious threat than, metaphorically, a child losing its baby-teeth.
I'd say it was more equivalent to an adult losing at least a leg and maybe an arm too.
That's what made the Civil War a matter of national life & death.