That simply is not true. He was a p4p top three when he finished Serra, Hughes, and Penn and when he fought Fitch - and in all of those fights GSP used flashy moves. The only fight after the Serra loss where GSP didn't look so great was against Koscheck.
What changed GSP was the money. He made millions after Hughes 3, Serra 2 and Penn 2. Then he slowed down. You can see it clear as day. His TD's aren't as fast as they were, he doesn't mix up his striking with big combos anymore or rush in to finish guys after they are hurt. He just relaxes and keeps it slow.
The GSP that fought and got a UD against Fitch would likely have finished Hardy (with ground and pound mixed with sub attempts), Jake Shields (pouncing after hurting him) and definitely Koscheck in the 2nd Kos fight. (going all out after Kos clearly lost vision).
I don't blame him for not finishing Alves, he messed up his groin. And I don't blame him for not finishing Condit, that was a hard victory after a major injury - but he should have and could have finished those other fights. He didn't because being champ is cushy, and he's not as fast/as much of a monster as he used to be.