Possible vengeance from survivors of The Great Massacre wasn't out of the question for Michael. That's one of the reasons why he kept Clemenza, and later Pentangeli, reporting to him. They and their organization were his muscle--his deterrent.

BTW, in a deleted scene, Neri shows Michael a photo of Fabrizzio, and tells him that Fab entered the US in 1955, "sponsored by the Barzini family." That tells me there was one or more survivor(s) in the Barzini family with a link to the past, even if it was to make good on a promise Barz made to Fab.