But they had so much time to sell at 1,000$, then at 800$, 600$, 500$...
https://blockchain.info/charts/bitcoin-days-destroyed
Indeed, since the total transaction output volume is around 500'000 BTC per day, it follows that bitcoins that are being moved today were last moved 4 to 30 days ago, *on the average*.
However, if someone moves to an exchange a pack of 1'000 BTC that he acquired 1000 days ago (in early 2012), that would be only 1 million bitcoin-days destroyed. It would be practically invisible on that plot. And everybody knows that a 20'000 BTC hoard must be sold gradually --- and that one should never deposit into an exchange more than what is needed for immediate trading...
The point is that the "bitcoin-days destroyed" plot will not tell us whether old-timers are dumping or not.