In order to contain slavery to the areas where it existed. Keep slavery out of the territories and each new state created from them would be free. Each free state had two anti-slavery senators. New free states meant anti-slavery congressmen which would dilute the south's disproportionately high congressional representation. All would keep slavery penned up so that it would eventually die. So yeah, the Republicans looked forward to the day that slavery would die. They were just realistic about it.
So why did the southerners want to open the territories to slavery?
Slavery vs. wages.
Strong sentiment on both sides of the WBTS but nowhere do I get that the war was over the moral issue of slavery.
"Slavery is likely to be abolished by the war power and this I and my friends are in favor of, for slavery is but the owning of labor and carries with it the care of the laborers, while the European plan, led on by England, is that capital shall control labor by controlling wages. The great debt that capitalists will see to it is made out of the war must be used as a means to control the volume of money." Private circular of Northern banker, late 1861.