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. Whether or not an successful attack has been made against a PoS coin doesn't address the valid security criticisms of the framework and is false as well:
http://www.cryptocoinsnews.com/mintpal-gets-hacked-pos-vericoin-fork-result/http://coinjoint.info/navajo-suffers-double-spend-attack/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.