Let's say Satoshi was a group, it would seem there is only one spokeperson who posted on the forums, as the posts are consistent up until he or she or they disappeared. So maybe he was working with a group but the main character is the only one posting as a single entity.
But doesn't explain why they haven't moved their stash of BTC though!
There are two simple explanations for that:
1. Satoshi died.
2. He deleted access to the wallets / never had access to them / mining computer nuked / did not care for value.
It's quite possible someone has more than enough money to resist the urge trying to cash out a few billion dollars worth of bitcoin, but that's unlikely.