Satoshi's whitepaper referenced work that other people did, Satoshi didn't write it alone. I can't see Bitcoin being a government back project headed by a mysterious Satoshi. Who could've known the impact Bitcoin would have ?
The bitcoin Satoshi mined might move one day. If it happens it's going to bring new theories about who Satoshi is.
What if it's a government led project. I agree with you that hardly any team would be able to keep such a secret to themselves forever unless it is a government operation. I think it is at least a possibility that can't easily be ruled out. If it was a one man show, what do you think how long it took that person to write the whole code?
From today's perspective it might look easier than it actually was to pull the whole thing off successfully. I mean sure the ingredients were there but when you first sit down and try to make the right decisions and combinations to come up with something of Bitcoin's calibre, that is still extremely impressive.
Also the question whether Satoshi is dead or not, who would be able to refrain from touching a million BTC? Could a single individual resist? I don't know. A government could I guess.