Delegates are still voted in by stake, stakeholders could simply vote them out instantly if they are compromised (though good luck compromising 51 de-centralized delegates), Bitshares makes this a very fast and simple process. No attacker would be dumb enough to target the Delegates. They would still target the stakes.
Doesn't really matter how many stakeholders are hacked (just like Bitcoin, if you get hacked, you lose your coin), since the hacker is now the new stakeholder, even if a hacker controls 51%. The dis-incentive to attack is built in. Why would you attack the eco-system you hold 51% stake in? and destroy its reputation and value, and in the process destroy your own wealth??? it goes against human nature. (and again, good luck hacking enough stakeholders to get 51% stake, unless big stakeholders somehow are all morons, they will make it impossible for you to hack)
You are making the flawed assumption that delegates or stakeholders will be aware they are compromised. If they aren't aware than an attack can happen at any moment or could have already happened.
Do you understand that there are ways to cheat in a DPoS framework without immediately arising suspicion that the protocol is compromised?
You keep discounting the importance of hypothetical attack vectors when they are of utmost of importance in information security. All investors of "Bernard L. Madoff Investment Securities " were quite happy clients with no history of an attack until Bam a "hypothetical" ponzi security vulnerability hit them.