If Satoshi Nakamoto shows ANY sign of still being alive and involved in crypto, Bitcoin is INSTANTLY going to the moon. Everyone is gonna go totally crazy if he decides to show up, it won't even matter if he dumps BTC/BCH or whatever he does, just his presence will boost us a lot.
Well, it actually depends what kind of sign he gives. If you see his BTC wallet address going live, and you see him starting to sell coins, BTC would probably drop 80% in a matter of days (not because of his huge quantity, but people loosing faith in BTC).
It is hard for inventors to sell their coins actually. People need to liquify their investment, and just because they sell some of their tokens, doesn't mean they don't trust their idea (there was a huge hype when Vitalik sold a tiny stash of his ETH, probably just to cover his life expenses).