He is not what he used to be but there is no surprise there. We should not expect him to be the world's best GK at this age.
We should stick with him as long as he performs the saves that every good goalie can do without many silly errors.
I've been having this debate with my brother and my cousin who are both interisti. They think he is now a liability and that Juve should replace him (and they recognize he is one of the best in history). My argument is that he is as good as any goal keeper the top teams have. I had a list of goals conceded by todays top goalkeepers that are either silly mistakes or savable shots (goals that a WC goalie should save).
LLoris had tons of them, De gea even more, Nuer has some, Reina has some, Chech has the least, handanovic has one silly error. Buffon is now at their level despite being much older and that says a lot about how much of a great keeper he used to be. He commits (like them) silly costly mistakes and the occasional savable goals go through him but he also saves most of these savable shots and occasionally makes a world class save. Simply put: there is not a single goalkeeper today who saves all savable goals, pulls the occasional world class miracle saves and does not commit silly errors like the young buffon used to do.
Today if we replace buffon with De gea or Nuer or Valdes or Casillas or any of those (maybe except handanovic) we would barely notice a difference in quality.