What if boxing was governed by a law that stated all fighters had to give up the sport at the latest on their 30th Birthday? Who would of been the best? Who would benefited the most? Who would of suffered the most? But remember the rule. It in theory it sounds like someone like George Foreman is going to suffer, as his entire second career is no more. But remember Foreman is not getting beat in no Rumble in the Jungle (at least not with a 1974 vintage, Muhammad Ali). So there are benefits as well, for him...
Does anybody get within blinking distance of the Great John L Sullivan or Jim Jeffries? Louis is Kod by Schmelling quite brutally, Ali loses to Frazier, Marciano and Johnson barely gets going, etc. Unless i have got the dates all mixed up, Holmes might be the best chance to get near them.
Louis does not get KO'd by Schmeling, Max was past his 30th birthday, when he beat Louis. Holmes is out of the game in 1980... Like you say, Marciano is burned bad.
My top 15 lineage Heavyweight Champions under the law: 1: Joe Louis 2: Muhammad Ali 3: Joe Frazier 4: Jack Dempsey 5: Jack Johnson 6:George Foreman 7: James J Jeffries 8: Max Baer 9: Mike Tyson 10: James J Corbett 11: Floyd Patterson 12: Sonny Liston 13: Rocky Marciano 14: Larry Holmes 15: James Douglas