Would it have been horrible if the slave states hadn't seceded for no better reason that to preserve their right to slave labor?
Please don't dirty this issue by comparing it to that.
Except, they didn't. If anybody thought even the original 7 seceding states' primary concern was the preservation of slavery, Lincoln and the Northern dominated Congress passed (with the necessary 2/3rds supermajority) the Corwin Amendment which would have enshrined slavery expressly in the constitution and made if effectively irrevocable. The original 7 seceding states turned down the offer of slavery forever in exchange for returning. The Upper South did not even choose to secede until Lincoln chose to start a war to impose federal government rule over people who no longer consented to it. Please do not try to revise history to argue otherwise.
That is a lie that has been used for propaganda since the war started and it is told for the purpose of misdirecting people into believing that invading the southern states was justified.
The Northern states passed the Corwin amendment which would guarantee continued slavery in the United States indefinitely. Furthermore, slavery was never under any threat of abolition from the Union until it became advantageous to the war effort to pursue it.
When Lincoln started his invasion of the South, he had absolutely no intention of abolishing slavery.
What the Southern states wanted was economic control over their own trade. You may not be aware of this, but the southern states produced 73% of all the trade with Europe.