Fedor and Roy Jones Jr. have had very similar career paths.
Fedor was a beast in his prime, but his biggest weapon was his speed and insane reflexes. He never developed a sound defense just like Jones Jr. never did, and once they started slowing down just a bit, the other guys started landing bombs cleanly.
I think Fedor was one of the greatest mma fighters ever, but he deteriorated much faster than others because he didn't take good care of his body and because he always had such a huge advantage in speed he never developed his stand up technique.
It is impressive what he has accomplish when you consider the way he fights. His hands are always way down, and he just throws bombs left and right. Other fighters know whats coming, he never threw straight punches but he still caught them because he was cat quick.
Fedor was relatively good at his time, but I don't think he deteriorated. He's just as good as he ever was. Same is true of Cop and Nog. The sport merely passed them by. And it all happened years ago. Only dodging and ranking tricks have kept Fedor alive so long.