“What would happen if slavery “expanded” into Kansas, and Nebraska, and other western territories? (where it couldn’t really expand because you couldn’t make plantation farming work in any of these territories)”
Great question.
There was considerable fear among abolitionists about the expansion of slavery into the western territories.
In 1848, the Mexican Cession territory included all land from TX to California. Its status regarding slavery was the leading major issue nationally. Had the Democrats prevailed, there could have been slave states from the Atlantic to the Pacific.
Spanish and Anglo landholders had been enslaving Indians there for generations.
While there was only about 24 slaves brought from the South in the territory, and plantation farming was unlikely, it was a hot topic.
In 1859, New Mexico passed an elaborate slave code that permitted the owning of African-American slaves by former Mexicans and supported the rights of all slave owners.
Lots of emotion, little reality. Cotton won't grow in West Texas, New Mexico, Arizona or Southern California without modern irrigation systems, so it wasn't really practical to have slavery in the territories.
But I believe a lot of people didn't know this at the time. (The slave plantation people probably did though.)
I regard the debate over slavery in the territories to be much like modern debates on Abortion or Homosexuality, where many people opine, but few actually know the facts of the debate.
People may have been ignorant, but they still had opinions.