mr nakamoto
-could have been already wealthy enough before inventing bitcoin
-could be more more than one person, which broke up/having a fight
-could have lost access to priv keys
-could have destroyed priv keys
-could be a secret service project that not even the secret service knows about and already cancelled/finished/aborted
-could be dead
-could have had an accident/sick/stroke/amnesia
-could be in jail
-could have decided to live as a hermit in the taiga
-knows what panic it could cause if he starts to move the oldest coins
-and so on....
since he created bitcoin to replace the fiat banking system, why would he dump his precious coins when fiat is weakest and his coins are strongest ? he might not be thinking in terms of fiat accumulation, but in terms of revolution. revolutionaires tend to be less corrupt then average.