I think that if Tessio had succeeded in setting up Michael to be killed, Barzini would have made sure that Clemenza, Neri and Rocco would have also been taken out at the same time.

Most likely, to play it safe, Fredo would have been taken out by Moe Green in Las Vegas because he had already "tocked to Barzini."

And while Barzini probably promised Tessio that he would make him run what was left of the Corleone business, I really believe that Barzini would have had Tessio killed too. After all how could he have trusted someone who turned traitor on a family that he had been a part of for so long? You can never trust someone who is willing to turn traitor...even if, for that moment, they are on your side. Once a traitor always a traitor. So Tessio would have been a goner too. It would have been the smart move by Barzini to get rid of him in the end.