Not everyone has dismissed the idea. I can show you right here on this board - post 562 for example - where a very strong advocate of Lincoln's War stated the North had, what was called, “moral high ground” compared to the South.
Let's look at the actual claim: “The South launched a bloody and protracted rebellion to protect their institution of slavery. Just about any position is a moral high ground compared to that.”
And so, critic answers critic.
And where did I say the moral high ground for the North was opposition to slavery? You keep making things up as you go along.