I think if the murder of Michael (and no doubt Clemenza) had come off, Tessio's days would have been numbered. Barzini would have no further use from him, and nothing is as untrustworthy as a caporegime who betrayed his Don to another Don.
... which raises the question whether he really was the smarter one. I mean, he knew the game. Traitors rarely survive, do they?
An alternative move could have been that Clemenza and Tessio form an alliance and kill Michael, Tom and Neri. Then make a deal with Barzini. Risky but not impossible.
