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Unless they decide to fight each other, the Klitschkos can be grouped together. It doesn't matter who either of them fights right now, the opponent will always be a step down in competition.
Their have demolished the heavyweight division to the point that they are fighting little-known fighters, who have a better shot of winning the lottery than beating either of the brothers.
Vitali is 40 years old, and Wladamir is 35. Vitali has a shorter way to go, but I would put the odds about equal. Both fighters are more likely to be stopped by an injury, than by an opponent in the boxing ring.
It's unfortunate for boxing that all of the best heavyweights are playing other sports. I reckon that the NFL has at least 30 players that if trained properly, could take down either Klitschko.
Right now the brothers have the best chance to beat Hopkins record, as they have literally no competition. I'd like to note that I'm not writing off Povetkin, he's a good fighter, he just doesn't have the physicality or the style to take down the Ukrainian champions.