After Vito's death,the Corleone's future lies in the hands of only one man, and that man is Michael. No Mike, no Family.

Forget Fredo,and neither Tessio or Clemenza would have the ability to take the Family in the direction it needed to go. Even if they ventured into the casino business,Roth would have succeeded with them where he failed with Mike.

None of the other possible successors would have the experience or respect to take over. No matter who tried to take the top seat,they would probably try to make Hagen an ally.Given all the possible turmoil,he might just say "the Hell with this" and march down to Wall Street with his resume.