I know that we are talking the Trilogy here, but in a way, the novel "kind of" answers your question.

If memory serves me correctly, after the great massacre, there were several Dons left who were not killed by The Corleones (can't recall off hand which ones right now) along with the survivers of the family heads that were wiped out. And I think that the novel says that the surviving Dons made the peace with the Corleones and that those families whose Don's were killed were split up and most of them made deals and were taken over by The Corleones.

So to answer your question and going by the movie, if Barzini had survided, but ALL of the other Don's were killed, it is my opinion that Barzini might have made an attempt to try and take over what was left of the families. He may have tried to get what was left of the other families to align with him against Michael, but I don't believe that he would have been successful in trying to do so. Just as the surviving families were unable to take on Michael in the novel without Barzini, Barzini would have been unable to defeat Michael without the other families in the movie.

Anyway I think that Michael probably had a "plan B" already in place had his attempt to murder Barzini failed.