Everyone can play well for half of a season or season or two, but where's the consistency? That's the sign of a great player and when we're looking at the Dybala, i haven't seen the consistency that it takes to call him a world class. He might fit the category of world class technique wise, but that's about it.
Still got time to show, but im pretty sure he is past his prime. I just can't see any reason why he would be better when physically declining, since he has always been quite poor physically. If you take even little bit off from the speed, recovery etc. it just seems there isn't anything left for him to adapt in order to compensate decreasing physique.