I don't think it has to be either or. Throughout history, we find examples of events that people try to oversimplify, and this is another case. I think there were many reasons why the Civil War happened. Tariffs/taxes was only one. Slavery definitely played into it as well. But, IMO, more important than either was the age-old instinct, if you will, of the State to exert control. Lincoln's actions during the Civil War make Bush look like a wannabe, in terms of the sheer level of unConstitutionality (aka illegality). It was a Federal/executive power grab, pure and simple - a smack down of state sovereignty which was cloaked as the liberation of a people.
I do think it is typical for people to imagine that somehow, absent the Civil War, slavery would have persisted forever, which I think is ludicrous. Spilling that much American blood demands that people believe this, I suppose. But I think it likely that slavery would have been abolished by peaceful means within a couple of decades if Lincoln had not gone to war. Then again, I have not done a lot of due diligence on this aspect of it, so I could be off base in this assertion. Hopefully, some folks more knowledgable than I will chime in.
None are more hopelessly enslaved than those who falsely believe they are free. - Goethe
Canada and other British colonies ended slavery without a civil war; given time America would likely have done likewise. From my perspective the US civil war was the result of a federal power grab.
gab in Canada
At the time slavery was abolished in the British Empire the British Colonies in North America were mainly agrarian. When you have one short growing season in a northern climate it was probably not economic to house and feed many slaves through the long non-productive seasons. It seems that slavery was much more profitable in subtropical and tropical climates with longer or year round growing seasons.