You would think Satoshi would destroy the Bitcoins rather than destroying the private keys. Private keys can be copied, Bitcoins can't, destroying the private keys is a bad way of destroying Bitcoins. Data forensic techniques could be used to recover the deleted private keys, the private keys could have been compromised in the past, or there might be a backup he forgot to destroy, and there are some cases where you cannot guarantee the backup has been destroyed (for example, if you were storing a backup in the cloud). Satoshi knows this so I think if he wanted to destroy the Bitcoin he would do so via some other method rather than deleting the private keys and hoping they are actually deleted.

Also, we actually don't know how much Bitcoin Satoshi has. There are various theories based on various assumptions and pieces of evidence, but nobody has produced anything conclusive. For example,  1,000 Bitcoins mined in 2009 that were once thought to belong to Satoshi didn't belong to him at all and the real owner was tracked down. It's possible that Satoshi may not have as much Bitcoin as we think he/she does.