He could tank the price to $1000 and still be a billionaire. There's a much more plausible reason for why Satoshi's coins haven't moved.
Who would realistically not want to cash out?
1. Someone so purely interested in the tech and not money they'd give up the wealth 2. Governments, specifically the ones that don't consider a few billion to be a lot 3. Someone who's dead
It would also seem likely that if he at any point was alive and realised he wouldn't be able to touch his original wallets, he'd still get in early enough to be rich from subsequently crested wallets nobody would suspect.