Rocky Marciano - proof that if you train hard and stay determined anyone can be great?

Rocky Marciano was perhaps the most well conditioned of all the heavyweight champions. After a poor performance in an amateur fight due to being in bad shape Marciano vowed never to enter a boxing match in bad shape again, and from all the fights I have seen of him he never did. He had extraordinary determination and would live like a monk in training, he would do nothing but eat sleep and train for months leading up to a fight, after a fight apparently he would be back training within days. However I don't think anybody could be great, Marciano did have his share of disadvantages as you say but he also had crushing punching power, a seemingly unbreakable will to win and a solid jaw. Marciano isn't proof that anyone can be great with training and determination, however he is proof that training and determination can help you overcome your disadvantages.