Originally posted by Turnbull:
If Michael had suspected Rocco of complicity in the Tahoe attack, he would have had him killed ASAP. Unlike Fredo, Rocco would have been too dangerous to let live. Rocco had simply outlived his usefulness to Michael, and he was expendable.
The other possibility is that he'd leave the status quo intact, so that the conspiracy would continue to operate and he could determine its full extent. Then he could get rid of all traitors at the same time.