The Yankees never would have allowed anyone to take even one southern state.
The land was too valuable. Approximately 90% of the federal budget at that time was coming from the south.
I can prove that at least 74% of all taxes were paid by the South. (Which was only 1/4th of the Total US population.) I've seen others who using Department of Commerce records, claim the total was around 83%.
So the realistic range of what level of taxes the South was paying should be between 74-83%.
The South was also paying for the bulk of all the commercial shipping in the United States, on the order of 90% of it.
New York got very rich from handling the South's exports to Europe. Washington got very powerful taxing the European imports to the US which were paid for by those Southern exports to Europe.