I'm not doubting that Satoshi's earliest mined coins that were worth billions at one stage were left untouched but couldn't it be possible that he still sold a couple million dollars worth of BTC and is living the high life today?
Certainly, Satoshi could exchange some of his bitcoins without arousing suspicion just as you say. However, I expect he would be noticed long before reaching millions of dollars (assume USD) worth of BTC. It is believed that almost all of the unspent coinbase outputs belong to Satoshi (most everyone else combined their outputs, creating large balances at a single address). On top of this, there's a
strange pattern in the nonce values of the old blocks with unspent coinbase outputs which allow us to decide coins owned by Satoshi with even greater accuracy.