Basically, Vito didn't seem to have a choice. It was Tom or nothing.
That's the long and the short of it.
Obviously, succession-planning was not Vito's long suit.
I wouldn't say that succession planning was Vito's weak point. I would say his reproduction skills were. He had one kid with a hugely hot temper, another who was a wimp-weasel, and the only kid fit to be Don was ear-marked for legitimacy.
