no, really because he knew that after he was gone, that there was a good chance that one of his two capos would turn and he needed someone he could trust to be there for Michael.
And since he didn't know which one or maybe even both, he started a new loyal one that no one knew about.
Now he couldn't just start a new build up regime that everyone could see or it would be giving others a reason to come against the family.
Basically this, remember Vito had said that he would not be the one to break the peace at the Commission meeting. Rocco's secret regime (if made un-secret) could be interpreted as the Corleone's moving to a war footing something which Vito wasn't ready to happen yet.