Perhaps the northern states didn’t think the issue through that thoroughly. Perhaps they simply assumed that any state in which slavery was legal would inevitably grow to become a slave-based economy and would therefore side with the existing slave states in all political matters. As the professors point out, the difference between the north and south was over many issues, not JUST slavery, but the divide was roughly the same as the mason-dixon line.
I suppose an analogy to today might be Puerto Rican statehood. Most liberals support it (because Puerto Rico would almost certainly be a blue state), and most conservatives oppose it. I’ve already heard talk from many neo-cons that if Puerto Rico becomes a state, we’d have to create another red state for fairness/balancing purposes. Slave state/free state seems to me to have been the 1850s version of red state/blue state.