It was already in place. Lincoln offered to support the Corwin amendment which would have kept it even more firmly in place.
So since Lincoln was saying "If you like your slavery you can keep your slavery.", then how could the fight be over slavery?
Seems to me that the fight had to be over something else, because Lincoln offered protection for slavery before the war even started.
It is true that Lincoln expressed no desire to end slavery in the South. I think it was a practical position for him to take at the time. Even many Northerners did not care about slavery in the South, they just didn't want it in their back yards. His main aim was preventing the spread of slavery out west and slowly increasing the political power of free states (over slave states).
Good for you. You’re still evolving, I see. Just earlier today you had stated that the Corwin Amendment would have made slavery permanent and irrevocable in the Union. Seems you’ve softened that a tad and now you say the Corwin Amendment would have “kept it more firmly in place”. Hopefully, by day’s end, you will realize that the Corwin Amendment was all and only about making slavery a States rights issue, and not an issue that Congress or the Feds could have any hand in. That would be real progress on your part.