It was what it was. The Union was victorious and we were better for it.
The only positive that would have come if Lincoln had let the south go (as many New Englanders wanted anyway) is that Cuba would have likely been annexed by the Confederacy. Therefore, when, by the 1870s and 1880s, when the Confederates would rejoin the union (due to high inflation caused by the printing of fiat money, and the lack of a coherent national economic policy in general, to say nothing of the need for industry), Cuba would have been a part of the United States.
And look how the tables have turned. Many of us, in the South, midwest, mid-atlantic, and west, would gladly give New England away.
(part sarcasm)