I should clarify this by saying "if we take Lincoln at his word (difficult, given his track record--but let's say) that the war was not about abolishing slavery but rather preserving the Union, what purpose did it accomplish?" How can it possibly be worth the cost?
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.