I reckon Liston would have stayed champ for another four years, beating Cleveland Williams, Zora Foley, Eddie Machen, Ernie Terrell etc until he was an old man and battered by a young Joe Frazier.
Frazier would ahev stayed champ until he ran into Foreman who would have smashed everyone - Spinks, Norton etc until he ran into a young Larry Holmes in about 1978-79.