I firmly believe that even if real satoshi turned up one day, people will also declare him Faketoshi.
This is so true.
Even if Satoshi presents his privatekeys, signs messages, shows his passwords, e-mail account accesses... it could be someone else who has all these privatekeys, passwords, accesses now.
He would only prove the ownership of these but it is impossible for somebody to prove that he is Satoshi.
Suppose Satoshi has lost all his private keys and passwords, or he has forgotten everything or some else has hacked into his computers and taken over it then your assumption is correct but Satoshi knows how he will prove his identity if he wants to prove it. Finally, if Satoshi plans to reveal his identity then it is up to him, when and how he is going to do it. Isn't that true? I do believe that many people in the Bitcoin community have met the real Satoshi Nakamoto but they won't like to tell it any one because of security reason.