It's, I think, sort of true that the Civil War wasn't always going to necessarily mean the end of slavery if the north won.
It started as a war to keep the union together, and initially a lot of people in the north thought that it would end quickly and that the states would return to the union and give up their rebellion.
However, as time went on and the losses started to pile up, it became clear to Lincoln and the other northern leaders that a war with this much bloodshed must end the slavery debate for good. That is why Lincoln ultimately wrote and delivered the Emancipation Proclamation.
But it's a point that's splitting a lot of hairs and very nuanced, because the Civil War started when pro-slavery states seceded from the union because they were afraid that a president elected without consent from any of the southern states might move to eliminate slavery...so summarily, the Civil War was definitely about slavery from beginning to end.
I’ve found this quote from Lincoln to be illustrative.
“If I could save the Union without freeing any slave I would do it, and if I could save it by freeing all the slaves I would do it; and if I could save it by freeing some and leaving others alone I would also do that. What I do about slavery, and the colored race, I do because I believe it helps to save the Union; and what I forbear, I forbear because I do not believe it would help to save the Union.”
I also find the fact that the Emancipation Proclamation did not free slaves living in border States, but only States that seceded to corroborate this.
That being said, Lincoln had long been know to oppose slavery and supported its abolition.
This is a bad take. The trend in the US and elsewhere was abolition. One reason for secession was the South could see the writing on the wall. Lincoln's election being the most obvious sign.
We could imagine the South losing the war and keeping slavery, but only for a short while.