Joe Louis wouldn't have been near the household name had WWII not happened. His lengthy uninterrupted reign of heavyweight champion happened pretty much solely because of the war. Who knows how many guys could have come gone before Marciano came along in 1952? Conn or Wolcott might have even been able to take him in the 40s.
But if WW2 never happens, he likely doesn't come out of retirement to fight Wolcott in the first place.
If Archie Moore had gone off to war he might never have had perhaps the greatest comeback in history. And a huge reason Muhammed Ali got into boxing was because of Archie Moore's fights.
Sports is so wild at the best of times it's impossible to say, literally anything could happen with a tiny POD, let alone a big one like going off to war.