What you conveniently forget is that slavery always ended due to government action, either through legislation or buy-out or something along those lines. And it was always done in the face of strenuous opposition from the slave owners themselves. And it overlooks the fact that it died out in Haiti somewhat suddenly and very violently.
...that it died out from north to south as it was doing here.
Slavery was a colonial issue for Great Britain and Spain. To say that it died out 'north to south' shows little understanding of geography. Slavery in Brazil was more of a 'west to west' thing since it was mainly along the coastal plain. Slavery in the American south wasn't dying out period. It was very profitable, had grown between 20 and 25% in the decade prior to the war, and you would be hard pressed to come up with a quote from a southern leader who thought it was dying. If it was, then why protect it in your constitution and why launch a war to defend it?
Most northerners did not want the war to continue, or was that riot in New York from people storming the army recruitment centers trying to get in?
And yet the Northern armies never melted away like the southern armies did. The Northern armies didn't have to resort to extending enlistments, or depend on conscription to the extent the south did. In 1864, when the 3 year enlsitments ran out, the overwhelming majority of the Northern army could have gone home but they didn't. Why not?
The way that line reads you know it does more for slow manumission than it does for expanding or continuing slavery. The only new slaves could come from the US and if the US was for ending slavery within its borders, at least that's what our northern history books tell us, there would be no more slaves coming in to the Confederacy.
Secondly, considering what was done at the draft riots and the position the north was in post-Gettysburg, would you go AWOL and with the consequences exhibited in NY to citizens, would you even want to?