It was all about slavery. Even Rhett's own letters, (not his public pronoucements when he was trying to hide slavery as an issue), along with the letters of virtually all the other secess leaders, emphasize slavery over and over again. You don't even get to the tariff as an issue really without slavery.
That's not true. You got to the Nullification Crisis without slavery, and that was over two things: the Tariff and States' rights under the Constitution.
Slavery was a wedge issue. Because it was the prop of the latifundist planter economy and the huge cotton market, of course attacking it meant attacking the entire Southern economy; and so of course people like Rhett got excited about that (who wants to be impoverished by a political movement?) -- but the point of the wedge issue, as Rhett explains in his address (if you bothered to read it), was to "consolidate" political opinion in the North for a concerted assault on Southern resistance to an industrial policy, the Morrill Tariff (which was more than just a tariff), and infrastructure improvements in and exclusively for the Northern interests but paid for largely by Southern and Western tax revenues.
The Civil War was about Northern businesses punking everybody else and making the rest of society their chumps. It worked, as shown by the manifest abuses of the Gilded Age.
And no, this is not a case of post hoc, ergo propter hoc; the Civil War really did enable, and Lincoln intended it to enable (he was a Whig, after all), the industrialization of American society that followed the Civil War.
That includes the proletarization of American labor, rampant wage-breaking, time-clocks and other forms of industrial tyranny, urban political machines, factory towns, and of course the protective Tariff.