Because all his coins are coinbase they are in 50 BTC chunks across anywhere between 10,000 to 20,000 different addresses and private keys which could be written down and stored anywhere. All with no history and total anonymity. He can cash in from one address at a time, over years, starting with the latest and get a fantastic income without tanking the price.
There are a lot of old blocks that were never spent that follow a pattern that identify Satoshi-mined blocks with a pretty good degree of certainty. If even a single satoshi of those blocks is moved, people will notice.
It would be interesting to see if anyone is actively watching transactions to see if any coins from the first few hundred blocks get moved. I haven't seen anyone posting about running a script for this.
No doubt Satoshi would be hesitant to spend any of the early mined coins, due to the risk of someone tracing back those transactions to an individual. If Satoshi wants to remain anonymous, Satoshi won't be able to spend the majority those coins.
That's all assuming those private keys haven't been lost or purposely destroyed.
