He asked about Top 15, not Top 20 and I don't think Walcott and Schmelling would be a consensus Top 20.
This is because Louis's best opponents didn't stick around to compile resumes. When a Louis number 1 contender was defeated they either retired shortly after(often in their mid 20s) or went down to the regional level like Mann and Godoy. But your Braddocks, John Henry Lewis's and Farrs were top 15 talents. Byrd is incredibly overrated because he beat Vitali and held a cardboard belt the longest. Like on this list he should be in the middle of title opponents Wlad beat. Hes the 4th best American. Byrds like Walcott on Louis's resume. People put him above Louis's legion of better opponents because Walcott got 5 title shots coming off losses and won 1 and joined the club of immortals.
Agreed, it also did not help that in his biggest fights he failed to have exciting or compelling results 1) Ibragimov 2) Haye 3)Povetkin Were all significant fights put on HBO that got significant attention and he really failed to let his hands go and give a great performance. He also had such a poor showing losing the belt to Fury and that was a bad mark though I suppose its better than getting flattened. He did redeem himself in going to war with Joshua
Retrospectively, maybe. But nobody knew how fed Ali was at that time. And most of it was because of his own actions.