I think if Fredo became Don, it would be a fully disaster!
He hadn't had this "inner power" which only the Don and Mike Corleone have had. I think not even Sonny would be good enough to rule the Corleone Family, he had neither had this character like the Don. He was always too nervous. A Don must be calmed in critical situations, so that no one descries how he will act to solve a problem!
Michael was the only who had the skills to become the successor of his father.

Let never another know what you are thinking!