The fact is, the (Italian-)American Mafia is not all about revenge, in contrast to the Sicilians where revenge and "vendetta" is a common theme. Business suffers if wars are going to happen. Throughout the history of the Mafia in America, countless Bossess have been executed with little or no retaliation.

Luciano having Maranzano killed.
Gotti having Castellano killed and this one is a good example because a relative of Castellanos (who wasn't in on the hit) made no move against Gotti or his crew.

My guess is, if the Corleone execution of the 5 Families was real. Michael would have allied with the Capos of the Families, promising them power of their Family and then no one could oppose them.