Michael told Clemenza and Tessio that if they sat tight they would be able to have their own Families. Clemenza did. Tessio thought he couldn't survive until then. And ironically he was right!
But what would Michael done if Clemenza and Tessio had both turned traitor? Would the Corleone Family have survived after Vito's death?
Since at the time they were the only two family capos (possibly plus Lampone), their joint efforts would probably have toppled Vito. Working together or separately, they both had plenty of opportunities to murder Vito.
By the way, one of the most egregious errors that both Puzo and FFC made was to have the protagonists speak any words about forming their own families. There is nothing in Mafia history, anecdotal or not, that lends itself to an American Don or capo of a Cosa Nostra family being able to create a new Mafia family. While there is plenty of Cosa Nostra history about capos taking over families (a la Anastasia, Genovese, Colombo, Galante, Gotti), there is nothing that I am aware of that sanctions a Don creating a new family out of whole cloth.
