His decline had nothing to do with evolution of the game. In fact, HW is pretty much one of the most shallow divisions in the entire sport when it used to be one of the deepest.
CC had amazing striking, footwork, speed (which is what he lost the most of). He used to mix in his hands with his leg kicks, he had incredible TDD. And honestly, his defensive grappling was always good enough to get him back to his feet. He had such power in his legs/hips that he could force scrambles and get back up. He was rarely if ever controlled on the ground for large amounts of time except for against Fedor.
His decline came because of the loss of speed and wear and tear on his body after decade long of fighting. Many of which were years in where he fought 4-6 times. That can take its toll on anyone, even if they don't get hit a lot. Training will wear you down.