You are aware that when Lincoln sent that letter to Greeley he had already presented the first draft of the Emancipation Proclamation to his cabinet? And that the last line of that letter he clearly stated that his personal preference was an end to slavery?
Which means that had the South offered a conditional surrender contingent upon keeping slavery, Lincoln would have likely taken it at this same period of time.
I've seen it asserted that the Emancipation was primarily a tool for weakening the South's attempts to get foreign support and to boost the moral of his supporters, plus laying the political ground work for stealing all the money the South invested in slavery by taking them without recompense.
And yet, Lincoln permitted New Mexico to choose whether to be a slave or free state, in spite of the Missouri compromise. Nor were a few Northern states not slave states after 1865, although I forget which at the moment.
I come from stock on both sides.
I now know, however, that what I was taught about the war was true only after a fashion. Does that mean I would have or do support slavery? No.
The argument to me is moot. WaPo wrote in 2013 that there are 60,000 slaves in the US and 30,000,000 in the world. This was in 2013.
Our own debt makes us slaves to the debt holders, for which we shall pay dearly.
In short, the war is not the one of the 1800s US. It’s here and now.