Why? What were those slaves going to do to earn profit in the territories? It certainly wasn't going to be earned by growing cotton or tobacco.
Lincoln could say this because if slavery was confined to the south, it would die. Like communism, it had to expand to survive.
Why would it die? Was it going to get outvoted? Takes 3/4ths of the states to pass an amendment.
With 16 slave states in the Union, it takes 48 states to vote in an amendment over their objection. This would require a Union of 64 states which we still don't have.
So how was it going to die if it didn't expand?
I've heard this said all my life, but when I look at the facts, they seem to contradict this claim.
If Lincoln's efforts to pass the Corwin amendment had been successful, slavery would have continued indefinitely so long as a single state wanted it.
You never needed an amendment prohibiting slavery to kill it.
Merely appointing fed postmasters who would admit abolitionist literature in; appointing fed judges who would free slaves in freedom suits (such as Dred Scott); or appointment of fed marshals who would not pursue runaways.
These would have opened the door to reluctant but “forced” manumission.