Well, DT, I don't think that we can accrue the reasoning you express to all of the Dons. Barzini had a vested interest in the Corleones' denouement, but with that went the loss of protection. Also, I wonder if they had reservations about Barzini replacing Vito. As the leading Mafioso in the country, Vito was a known commodity. However, did they feel the same way about Barzini assuming that position.

In addition, I don't think the Dons had any interest in eliminating Michael as a potential successor to Vito. Vito's warnings at the meeting were directed at Barzini and Tattaglia to preclude any attempt at revenge.