It comes down to 'independence' and 'defending slavery'. Independence brings up the subject of states' rights, which Lincoln pretty much obliterated; the South became a plundered war zone for decades.
Slavery almost certainly would have declined over the next half a century anyway; historically that institution was destined to minimalize.
Who can say what the outcome would have been if Lincoln had not decided his principles merited spending the lives of tens of thousands of people. Depending on ones POV, Lincoln was either a stalwart or a monster. As I say, historians write the text of any story.
I'd argue it would have declined even faster. Lincoln could have done nothing in which case there is simply no way the original 7 seceding states could have defended a 1500 mile border from the Atlantic Coast of South Carolina all the way to El Paso Texas. With them out and now a separate country, the remaining states of the US would have had no obligation to return their escaped slaves. The result would have been that their slaves - especially the young and fit - ie the most productive ones - would have poured over the border into the US causing the rapid collapse of slavery. Something very much like that happened in Brazil which caused slavery to collapse in 2 years in that country. Lincoln made this argument by the way and nobody could really refute it.
Even if you don't believe this would have doomed slavery fairly quickly, industrialization was wiping out slavery in Western country after country in the 19th century. By 1881, Brazil and Cuba had gotten rid of it and they were the last two. Already 50% of Blacks in Maryland were freedmen. In Virginia 25% of Blacks were. Industrialization was working its way southward already by 1860 and the same process that killed it in the Northern states was going to kill it off in the Southern states too. There's no reason to think that slavery would have held on in the Southern states for 50 or 100 years longer than it held on anywhere else in the West.
Who can say what the outcome would have been if Lincoln had not decided his principles merited spending the lives of tens of thousands of people. Depending on ones POV, Lincoln was either a stalwart or a monster. As I say, historians write the text of any story.
Lincoln claimed he was "saving" the Union and that the Union would be destroyed if the Southern states left. Was that true? Remember that North Carolina, Virginia, Arkansas, Tennessee and Missouri were quite content to remain in. The union would still have stretched from Maine to the border of South Carolina and as far Westward as the Pacific. It still would have been a huge country. It just would have been smaller. The idea that there would be Three democratic Anglo countries north of the Rio Grande rather than just two no more horrifies me than the fact that there are two democratic Anglo countries north of the Rio Grande rather than just one.
Are you horrified Canada is not part of the US? Why not? Because you've been taught not to be horrified by the idea. Being part of the US is not what the people of Canada wanted. *shrug*. I'm cool with that. Why should it be any different for the people of the Southern States?
I'd argue that if anything, had the seceding states been allowed to depart in peace, it would have been GOOD for the US. Why? Because with the right of unilateral secession affirmed, it would have prevented the federal government from becoming too oppressive or rapacious toward any single state. They would have always had the option to just leave if they were getting screwed over too badly. That would have been a GOOD thing. That's what the Founding Fathers intended.