We don't know that Walcott out boxed him for 15 rounds because we haven't seen the full fight, and ten ringside sportswriters voted for Louis ,another gave it a draw. The comparison is that both Young and Walcott fought defensively.
Sure, it's not totally definitive being that we can't view the fight in its entirety. However, we can make logical deductions in the absence of definitive proof, and that is what we do here. When almost double the newspapers.. the ref.. the crowd all give it to Walcott, I'd say that's pretty solid proof. Then when you add in how the fight was scored (not under our current system), and realize had it been scored as we do today, it would've been decisive for JJW. Point is, we can try and masquerade around the fact that Joe Louis didn't officially lose, but the reality is, he very likely did.
Really? That's a bride too far for me. If you don't count those victories, he doesn't reach the top of the sport for me. No way.