No PoS system that I'm aware of, has actually been attacked, all the theories remain theories, the real world has said "no I can't attack a PoS system".
Many PoW systems have been attacked, the real world has provided many successful attacks, lots of PoW systems have basically been attacked to death.
How can anyone still claim PoW security is superior to PoS?
PoS may not be a perfectly secure system, but it's clearly superior in a security sense and also economical sense.
PoS scales beautifully, while PoW struggles to waste more hardware and electricity, and transfers more value out of a crypto eco-system.
You have some flawed reasoning with regards to security.
1) Just because no case of a 51% attack has been successful with Bitcoin doesn't mean that Bitcoin is secure from such an attack in the future. The same reasoning can be applied to any PoS with NaS. When it comes to security, analyzing all
possible attack vectors is of utmost importance.
2) To only focus on NaS attacks PoS/DPoS critics are not accurately reflecting all the possible attack vectors in which these currencies are vulnerable to.
I.E... Some would consider Bitshares to be recently attacked with a "51% democratic attack by delegates" which decided to change BTSX from a deflationary currency to an inflationary currency and upsetting a minority group of investors who were sold on the idea of a deflationary currency.