It will definitely pose a threat not only to bitcoin existence but to his own life or their lives. When satoshi chose anonymity it was definitely for decentralization and also to stay away from government troubles. With the way the government is trying by all means to ban bitcoin I think they would have either place satoshi on prosecutions or would have offered him somewhat of a deal to regulate bitcoin.
Also there will be many bitcoiners that would doubt the decentralization of bitcoin because some will feel that he might compromise the whole system someday.
Bitcoin is open source, I wonder if Satoshi can tweak and modify bitcoins? I don't think he'll make it, even though he's the one who made it. If Satoshi gets caught, he will probably be in danger, bitcoin will be negatively affected by that news, but it won't be modifiable, or someone will control it if Satoshi gets caught.