I think you have to count Roth in on this fantasy, Lou:

In his plan to have Michael whacked, either in Tahoe or in Havana, Roth had to worry about succession in the Corleone Family for his own safety, since he'd be a suspect sooner or later. If Tahoe had succeeded, Pentangeli would be the fall guy and would be on the run. The Rosatos would take over NYC. Back in Nevada, Fredo would be the logical heir-apparent, and Tom would have supported him out of friendship and family loyalty. But I doubt that Neri and Rocco would have accepted him. A war would have broken out--just what Roth wanted to divert and split the family. Given enough chaos, the Nevada Gaming Commission, with Roth's prodding, would have yanked the Corleones' licenses. Guess who would have picked them up?

Had Michael died of natural causes or retired, Neri would have been the logical successor. He, not Rocco or Fredo or Tom, was Michael's choice. In a deleted scene, it's pretty clear that Michael gave him a piece of the Tropigala(?) that he took from Klingman. But, while Neri had force, I saw no indication that he had Michael's business or political acumen--or that Michael ever groomed him as a successor. The other families might have been forced to accept him, but the Corleone empire was based on Michae's plans for "legitimacy," not Neri's force. It would have withered under Neri.