That's exactly what Northern interests were scared of. After the Morrill Tariff which taxed 50% duty on iron products, 25% on clothing, and an average rate of 47%, the South countered by creating a free trade zone between itself and Europe.
Consider this prediction from 1860 by the Chicago Daily Times:
"In one single blow our foreign commerce must be reduced to less than one-half what it now is. Our coastwise trade would pass into other hands. One-half of our shipping would lie idle at our wharves. We should lose our trade with the South, with all of its immense profits. Our manufactories would be in utter ruins. Let the South adopt the free-trade system, or that of a tariff for revenue, and these results would likely follow."
The North needed the South while the South felt it did not need the North.
As Alexis de Tocqueville wrote 30 years earlier, only states with a vested interest in preserving the union itself would act to stop another state from leaving. He predicted they would also do so citing their motive to be that of preserving the union itself, though the real interest would be something else:
"If it be supposed that among the states that are united by the federal tie there are some which exclusively enjoy the principal advantages of union, or whose prosperity entirely depends on the duration of that union, it is unquestionable that they will always be ready to support the central government in enforcing the obedience of the others...If one of the federated states acquires a preponderance sufficiently great to enable it to take exclusive possession of the central authority, it will consider the other states as subject provinces and will cause its own supremacy to be respected under the borrowed name of the sovereignty of the Union" - Alexis de Tocqueville, Democracy in America
As for the North needing the south, the reverse was true. The south needed someone, if not the North then Great Britain or France or someone else. The south was in a dependent relationship. It did not have the finance or manufacturing or transportation necessary to run their own economy and they had made no attempt to create one. Had the south been allowed to secede peacefully then they would have changed from a integral part of the United States to a virtual colony.