Senate rules change slowly. If I understand correctly, it took 80 years between the first filibuster and the introduction of a cloture procedure (or 111 years between the rules that made filibusters possible and rules that made ending filibusters possible). And those early cloture rules didn’t do much to stop filibusters. It took another fifty years for cloture to become easier and filibusters to become rarer. Realistically, Southerners didn’t have much to fear, but in those days the country (and slaveowners in particular) cultivated wild and unrealistic fears.
I am afraid your private assurances would not have completely resolved the issues.
Nor do they now.