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pipe dream on your part, totally unsubstantiated by anything, other than your imagination.

If it's a pipe dream, it seems to have been shared by Northern newspaper editors, but in their case I think they saw it as a nightmare.

March 18, 1861, the Boston Transcript
"If the Southern Confederation is allowed to carry out a policy by which only a nominal duty is laid upon the imports, no doubt the business of the chief Northern cities will be seriously injured thereby. The difference is so great between the tariff of the Union and that of the Confederated States, that the entire Northwest must find it to their advantage to purchase their imported goods at New Orleans rather than New York. In addition to this, the manufacturing interest of the country will suffer from the increased importations resulting from low duties….The…[government] would be false to all its obligations, if this state of things were not provided against.

The Philadelphia Press 18 March 1861
"a series of customs houses will be required on the vast inland border from the Atlantic to West Texas. Worse still, with no protective tariff, European goods will under-price Northern goods in Southern markets. Cotton for Northern mills will be charged an export tax. This will cripple the clothing industries and make British mills prosper. Finally, the great inland waterways, the Mississippi, the Missouri, and the Ohio Rivers, will be subject to Southern tolls."

DiogenesLamp quoting:

"The difference is so great between the tariff of the Union and that of the Confederated States, that the entire Northwest must find it to their advantage to purchase their imported goods at New Orleans rather than New York."

If the alleged quote is even real (I can't find it), it could represent exaggerated fears of Northern Democrats that their erstwhile political & economic Southern Democrat partners would gain an upper hand over them.

But such fears were never realistic because no Confederate ever considered making the Confederacy a "free trade" zone, and that would be required to create an economic advantage to importing through, for example, New Orleans.
That's because as soon as they put any tariffs on Confederate imports, the tariffs would eliminate all economic advantage of using the Mississippi River versus New York based transportation.

DiogenesLamp quoting:

"Finally, the great inland waterways, the Mississippi, the Missouri, and the Ohio Rivers, will be subject to Southern tolls."

And any such tolls would make using Confederate ports for Union imports totally cost prohibitive.

Bottom line: these quotes in no way explain why Republicans wanted to preserve the Union, but they may help explain why Northern Democrats were willing, however reluctantly & hesitatingly, to support the Republican war efforts.