The preservation of slavery is a solid argument for letting the South go. Continued presence in a Union that could elect a Republican president meant that the slave states would surely occupy a dwindling minority status which would make their anti-democratic advantages all the harder to maintain.

Aside from that, there's not a whole lot. Some in the North were happy to be rid of the slave states, minus four, but permitting secession was considered by wide swaths of the white north as nothing less than the destruction of republican government itself. If the losing side of an election, and there always is one, can just take their real estate and go every four years then national government is impossible and the Union is consequently at an end for everyone. Part of the deal with having elections is that one agrees one may lose them. Doing otherwise would leave individual states or regional polities easy pickings for European empires or one another.

I'm unaware of any serious historiographical arguments advanced today in favor of allowing secession. Outside of that kind of thing, modern discussion would run afoul of the sub's twenty year rule.