I think its more to do with experience, training, and managing injuries.
Cain is done as far as I'm concerned. As someone who's 31 and undergoing spinal fusion next year and had an MCL repair, I can tell you my MMA hobby is done. I wouldn't be prepared to even attempt a camp for a fight. Cain has had multiple back surgeries, shoulder surgeries and ACL repairs.
If he comes back, I have no doubt he'll either look like shit, or aggrevate one of his laundry list of injuries and sit on the sideline again.
Werdum never pulls out of fights. He's never had major surgeries (to my knowledge) and he just went 5 rounds without a problem against a solid up and comer.
Werdum is in his twighlight 30's though, and reaction times and such will start to drop fast for him, but given his health and constantly improving game, he has another run at the belt in him I believe.
I'd be surprised if Cain has even 3 more fights in him. While he's not old, he's racked up more wear and tear on his body from gym time than anyone else. Second o ly to Khabib. Ha, and they are both at the same gym. Good things happen at ATT.