It could be argued that it was the preservation of the Union that would perpetuate the revenue stream.
Conversely, it could be argued, with respect to the Confederacy, that the preservation of States rights was also meant to perpetuate slavery.
None of those are mutually exclusive and, thus, to me, it becomes a "chicken-or-the-egg" argument that just goes round and round.
My personal opinion is that the principles put forth in the Declaration of Independence apply as well to the Confederacy as they did to the 13 colonies.
The Declaration asserts that people have a right to leave a larger union and create whatever sort of government suits them.
That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness.This is the principle that founded our own government, so I think it is rather hypocritical for our government to apply a different set of standards to a group of states which want to leave the Federal Union.
If the need to "Preserve the Union" is the higher principle involved, then how could we have broken from the British Union?
Speculation ends with Lincoln’s direct wording in the first inaugural:
“The power confided to me will be used to hold, occupy, and possess the property and places belonging to the Government and to collect the duties and imposts”.
He is speaking of the forts which are only needed to force tariff collection.