- He is afraid of beeing found when he spends one satoshi
- He forgot his wallet passwords
- The harddisk with the wallets crashed
- He is dead
(Most likely in that order)
I think he would have more than one harddisk with the wallets. He's either dead or not deleted the keys, if he was going to spend the coins he would have already.
it would have made sense to give Gavin a backup? lol .. who knows

That would have been a horrible idea. At the time that Satoshi left he really did not know Gavin that well and he had really not yet shown himself to be as trustworthy as he has shown himself to be today.
Also it is never a good idea to give someone your private keys unless you intend for them to take/spend the bitcoin that the private key controls
Satoshi must have planned for a scenario where his account access would transfer to someone trusted after his death. Like transferring a Gmail account where private keys are stored in 'Draft' folder.
You can setup your Google account to be accessible to someone else in case you don't login for a specified period.
I don't think satoshi would have trusted the security of google. I also believe that the threshold to allow someone to use an alternate method of accessing an account is small so satoshi would only need to not access his gmail account for a short period which would potentially give gavin access to the private keys before satoshi intended.
Also google (and probably the NSA/US government) would be able to access anything in the drafts folder)
I sighted Google account as an example of digital afterlife. Doesn't necessarily mean Satoshi used Google for this.
The point is, there are methods to allow someone else to get hold of your data after your death - IF YOU WANT TO.
So even if we assume Satoshi is dead, it doesn't necessarily mean that all his private keys are lost.