- 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)
Collectively "Satoshi Nakamoto" is waiting for mass acceptance before revealing himself and starts dumping coin in market.
If there is mass acceptance, dumps won't be necessary. He won't need fiat.
The time to dump is now, while he is a multi-millionaire and bitcoin's future remains uncertain.
Why would he dump now at this price? Though the future is uncertain I think there's a greater chance of it rising considerably than crashing, though ups and downs will likely occur. If, like you said, there is mass acceptance and he or any of us wont need fiat anymore then it makes sense to keep hold of them right now.