I agree, Turnbull. Tessio came to a realization that Michael was an incompetent Don and that Barzini would run them over if left unchecked. A great deal of what I theorized would hinge on Tessio's timeline and his planning. Did he have a plan in place if/when Vito was out of commission permanently? How long would he have after Vito's passing to act to preserve himself?

I hang my hat, so to speak, on the premise that Tessio wasn't as short sighted as to believe that Barzini would honor such a deal and cease his offensive against Corleone (and by extension, Tessio's) interests. He knew what kind of man that Barzini was, and that he would have to deal with him with a quickness if he is to survive. Out of that, a plan forms to not only survive, but to thrive.

His underestimation of Michael, his impatience and ignorance of Michael's master plan are his undoing. No one except Michael himself, Vito and perhaps Tom knew what the master plan was. Like Tessio, Michael knew what kind of man that Barzini was, and he knew that there would be no type of deal between them. If he could've let it ride as Clemenza did (as he was advised to do), then he would've been sitting pretty at the end of it all.