I feel like most of what I've read about Lincoln manages to avoid giving a direct answer to this question, which is incredibly disappointing, because it has to be the most interesting question about Lincoln. The debates help a lot in getting to this, because they make clear the context for what Lincoln told Greeley. Lincoln says about slavery:
I think that [Stephen Douglas], and those acting with him, have placed that institution on a new basis, which looks to the perpetuity and nationalization of slavery. And while it is placed upon this new basis, I say, and I have said that I believe we shall not have peace upon the question until the opponents of slavery arrest the further spread of it, and place it where the public mind shall rest in the belief that it is in the course of ultimate extinction; or, on the other hand, that its advocates will push it forward until it shall become alike lawful in all the States, old as well as new, North as well as South.
To Lincoln's thinking, preserving the Union without freeing the slaves wasn't one of the options. That option had been sealed off, and he believed that it had been sealed off not by him, but by the South and the apologists for the South. For those opposed to slavery, there were no more concessions left to give. Just maintaining the status quo and letting the slave states maintain the abhorrent institution was clearly not enough. The South had been offered that and much more. As Lincoln saw it, the only demand left was that slavery should not only be permitted, but expanded into the free states. For Lincoln, the political realities of slavery (he outlines these in a fascinating way in a letter to his friend Joshua Speed guaranteed the political ascendency in the South of the most intransigently pro-slavery faction. And that meant the inevitability of war.
It was not a question of just letting the South go, because the South did not want merely to go and be left on its own. It had been offered the chance to be left alone and much more, and that was never good enough. The last demand that was on the table was one that Lincoln would not countenance: not just that slavery be tolerated, but that it should be expanded and perpetuated. So the question of just letting things lie as they were was a false one. It was an option that the South and its supporters had themselves taken off the table, and to concede any more, even if it was possible, would do nothing but delay the coming catastrophe. Thus the famous sentence: "If we cannot live together as brothers, how will we live together as enemies?" Or, in other words, if we are already close to war now, why does anyone think that we will not be at war after we split into two countries?