Apparently neither Puzo nor FFC saw any need to introduce an underboss. In fact, FFC also eschewed a consigliere in II probably for the same reason. Those two positions just did not serve any purpose in the films' narrative. As a further example, it apparently did not serve Puzo's purpose to assign the Corleones more than two capos longterm.
"Generosity. That was my first mistake." "Experience must be our only guide; reason may mislead us." "Instagram is Twitter for people who can't read."
