After all of that is considered, you might want to put more blame on Grant. It was his strategy to expend lives at the advantage of superior numbers. You might say that Lincoln could have taken action against Grant, but you have to remember that Grant was winning after a long series of Union generals who were mostly failures.
So the final point is that the South could be left to secede without any trouble, but there's a lot of debate about what effect that would have had. Especially in terms of civil rights.