Not impossible at all. You have to remember that the average slaveowner had 5 or fewer slaves. Not all slaves worked in cotton fields. A large percentage were cooks, servants, maids, grooms, gardeners, and the like. Men like Thomas Jackson who owned as many as 10 slaves at one time would expect to take their chattel with them had they moved to Arizona or New Mexico. And slavery would have continued to flourish in Cuba, which both the Douglas and Breckeridge platforms advocated acquiring.
So Crittenden Compromise was mainly for symbolism only, do kick the can down the road, and avoid a war. The South was game, but Lincoln was opposed to any concessions to the South, symbolic or not.
Not only kicking it down the road but guaranteeing that nothing could be done with it once you reached the can again.
Slavery created a lot of problems for not only blacks but for many free whites who didn’t want to have to compete against slave labor along with humanitarian reasons. Wealthy industrialists had no use for slavery either, as they needed educated workers, and slavery created a bad business and moral environment for them.
The only place where slavery was welcome was where it was spectacularly profitable and where an entire economy could be built around it. Tolerating slavery strictly for the benefit of a few rich people who enjoyed having some house slaves was out of the question.
The best place to establish a new slave state in the territories was obviously Kansas, with its farm economy. When Slavery in Kansas crashed and burned, it was obvious that it could not spread anywhere in the existing territories.
Are we supposed to believe that slavery which was soundly rejected in Kansas with a favorable geography, was going to be accepted in a mountain state like Montana or Wyoming?
That idea is ludicrous and Southerners at the time knew it was ludicrous. That is why after Bleeding Kansas, they started really pushing to expand to Central America.