It's been addressed loads of times. It's a chimera. It doesn't exist in real life. PoS has been running long enough, with high enough market cap, so demonstrate it 's secure.
Bitcoin is still experimental with a market cap of 6-10 billion. Nxt has a market cap of only ~36 million for less of a history than Bitcoin which makes your claim laughable.
So 36 million isn't enough to motivate hackers? Of course it is. Nxt is under continual attack, eg from hackers scanning for weak passwords. If the core system was weak, it'd be broken.
The first of those was about an exchange getting hacked, not the currency itself. The second was about a transaction getting accepted and then orphaned, which is something that can happen in any distributed crypto-currency and again does not mean the currency's security is weak. It's happened in Bitcoin, for example. Neither have anything to do with "nothing at stake".
Additionally, any bugs or flaws within a POS or DPOS exploited by a single hacker can be used to attack the network regardless of the intentions of delegates or the distribution of the POS pool. with PoW there are real resources being used so even if a vulnerability is exploited to hijack a miner they will quickly correct the issue because they are losing money and hijacking miners all around the world that use different equipment , different pools, and different uplinks is a whole lot more difficult than finding a flaw within a wallet of hijacking a few large stakeholders computers.
Bugs can arise in any software. There's no reason to think they will take longer to fix in PoS than in PoS. If anything, it's the opposite. Bitcoin has shown itself notoriously slow to fix flaws. Transaction mutability is still not fixed, for example.