What prevents someone from stealing million BTC from any exchange ? I don't see how PoW prevents this.
With decentralized Asset Exchange like NXT trade.secureae.com problem of stealing from exchanges does not exist.
BTC pump of 2013 - 2014 shows that mined currency does not have to be sold and can be hoarded too.
I still don't see compelling arguments in favor of PoW.
I believe there's two points here:
1. With PoW coins, there's no advantage to keeping the coins online, while with PoS coins the exchange is motivated to store coins online where they can stake them.
2. With PoS coins, a major theft can put a huge chunk of mining power into the hands of a malicious actor. See the case with the Vericoin hack from Mintpal, where 30+% of the total supply was stolen, leading to a hard fork to revert the transaction:
http://www.coindesk.com/bitcoin-protected-vericoin-stolen-mintpal-wallet-breach/