The problem with that assumption is it doesn’t recognize the southern fear of a northern Senate majority, which they saw as fatal to their cause, in the long run. They saw admission of Kansas as a free state as the beginning of the end of their way of life. You’re right that many Republicans probably miscalculated on that score.
But it was a pipe-dream to think Kansas or points west were suitable for plantation agriculture and slave labor. By 1861, slavery was illegal in almost all European colonies and throughout Latin America except for Brazil. They were on the wrong side of history and geography.