There have been heavyweight champions retired still holding the title but came back only to lose like James Jeffries who was forced back by his pastor of all people to fight Jack “Galveston Giant” Johnson only to lose. He gained 100 pounds and was off for six years, no wonder.
Muhammad Ali was another to be off over three years from March of 1967 to October of 1970 and came back to lose in March of 1967 after several wins to “Smokin” Joe Frazier in their first of three fights with Ali winning the other two.
Gene Tunney was the next champion to retire with the title and first not to return to the ring. After a pair of wins over Jack Dempsey Tunney would fight once more against Tom Heeney and retire in July of 1928 with a 65-1-1 (48) record.
Rocky Marciano, 49-0 (43), was another to retire and not come back after stopping Archie Moore in September of 1955. Moore would go onto fight Floyd Patterson for the vacant title with Patterson winning in November of 1956. When Patterson lost to Sweden’s Ingemar Johansson in June of 1959 Marciano went to Italy to train for a comeback to fight Johansson but realized he couldn’t get back to what he was after retiring some four years before.
Lennox “The Lion” Lewis, of London, UK, retired in June of 2003 as WBC and IBF Heavyweight champion after a tough fight with Vitali Klitschko who was coming on strong until a severe cut stopped the fight with Klitschko ahead on all scorecards 58-56. Lewis was not eager for a rematch I am sure. He retired after this fight. He has since become a cable commentator.