I think this is one of the biggest fallacies I see spouted about the state of American boxing.
What does boxing skill have to do with football and basketball skills? They are not translatable whatsoever. Anthony Joshua and Wilder are probably the most athletic heavyweights in the division, yet Fury is clearly the best of the heavyweights based on pure skill.
Being an athlete in boxing can only take you so far. Ultimately it's about skill and the will to win. The 90s was the best era for heavyweight boxing imo and that was dominated by a British-Canadian for the most part.
The era when US heavyweights were dominant, there wasn't as much global participation in the sport.