IMHO, preserving the Union and avoiding a condition of constant hostility and intermittent conflict would have justified a much greater cost. You obviously disagree.
But I can guarantee that the only effect of allowing the Union to collapse would not have been the avoidance of the WBTS and its death and destruction. Many consequences would have followed, including with regard to world history in the 20th century.
I suspect you would agree that a world conquered by the Nazis or Commies would be even worse than today's world. And that is indeed a possible consequence of a divided America, as a United America played a major role in defeating both great totalitarian challenges of the 20th century.
Would a divided America have been able to do the same? It seems highly unlikely to me.
It's interesting to wonder what would have happened had the two split amicably, but I tend to think that the two nations would have re-joined after some time had passed or would have become very close diplomatically.
Clearly, slavery was a dying instutition and once it died out in the South (only a matter of a few decades longer, I suspect), there was little conflict between the two. There is just too much in common between the South and the North for centuries-long hostility between the two.
As far as the 20th Century goes, don't forget that the United States and Great Britain were obviously two very bitter enemies, even as late as the Civil War (Britain was very close to entering the war on the side of the Confederacy), but put all that aside for the wars in Europe, and are now best buddies. I tend to think that a similar course would have been followed between the CSA and the USA, though obviously this is all speculation.