Let's say that after his loss to Max Schmeling, Joe Louis's luck takes a turn for the worse yet, and he falls under the wheels of a model T Ford. So his last fight is his loss to Schmeling, but his remarkable run of form up to that remains intact. He has still cleaned out the top contenders in brutal fashion, with the sole exception of Schmeling. How is he viewed today?
Max Schmeling beats Braddock. The Nazis control the heavyweight title. Max Baer wins the return with Schmeling and becomes the first two-time heavyweight champion and a beloved icon. Walcott never gets a title shot. Rocky Marciano never fights Louis or Walcott. Maybe he never fights for the title. The first black man to fight for the title since Jack Johnson might be Floyd Patterson. Larry Holmes probably retires as WBC champ after the Witherspoon fight, since there's no significant record of defenses (Joe's) to chase. Michael Spinks never wins the heavyweight title. Joe Louis is the Ike Ibeabuchi of his era.
About Joe Louis: I think he would`ve been seen as a kind of wasted gem. A boxer in which the immediate question arises: What could have been if....? Just to mention a few other cases: What could have been with HW Billy Conn without WW2... a carefully managed Pete Rademacher, some 80s HWs without drug problems, Ike Ibeabuchi without mental illness or Corrie Sanders and Odlanier Solis in in tip-top shape etc... But I think Joe Louis would stand out as even way more promising.
He would be known as that really good heavyweight contender that nobody in the mainstream would remember after about 5-10 years. Schmeling, assuming he beats Braddock in a title fight, would be one of the most polarizing and significant figures in boxing history. Whether he liked it or not
Even though he hadn’t yet come to the title when he lost to Schmeling in ‘36, if Louis’ career had ended there, based on his performances prior to, he still might’ve benefited from very high praise and some heavy extrapolations in the realm of fantasy H2H match ups, ala Mike Tyson. His career prior to Schmeling screams a destructive machine on a one way path to the title - with a few ex HW Champs left well and truly slain in his wake. The loss to Schmeling might’ve been interpreted as a major anomaly and one that would’ve been duly rectified upon a rematch. Fortunately, none of this was left to speculative guess work - Louis actually came back with a vengeance, won the title, reversed the Schmeling loss in the most emphatic fashion possible and dominated the division for a long time. Far more impressive than any paper calculations and projections that still might’ve afforded him future successes in all potential but successes that he didn’t achieve in his actual career.