In the novel Michael says, "He's [Tessio] the most intelligent. And he did what seems to him to be the smart thing ... He sticks with me and gets wiped out;"
Why would Tessio have gotten wiped out if he stayed with Mike?
I think Michael is speaking in the abstract. His comment is predicated on what Tessio's understanding of the Corleone Family's situation to have been at the time he betrayed Michael. While ins truth he would not have been wiped out if he had stayed with Michael, in Tessio's mind that is what would have happened.
