LOL Cro Cop had just won the OWGP when he came to the UFC. Everybody expected him to easily win the title and dominate the division for at least 3-4 years.
And? Everyone expected Cain to continue to dominate the division, rather than getting "sea-leveled" by Werdum.
It doesn't mean CC wasn't past his prime. ~9 years is generally accepted as the career length of most fighters before age and damage really starts to set in, regardless of how old they actually are. The Wandy win wasn't even the biggest fight of his career; both his Nog and Fedor fights were bigger.
It was just his biggest achievement.