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.
Bitcoin is not a mining algorithm by itself, it uses the same PoW algorithm as many other PoW crypto, and since other systems with the same PoW algorithm has been attacked, therefore it's already proven Bitcoin can be attacked in the same manner. Bitcoin has the advantage of being an order of magnitude larger than any other crypto, that's another form of security, unrelated to PoW.
For example, yahoo and my personal blog site, both can be DDoS attacked, but yahoo being so big, it's much more difficult to DDoS it. It doesn't mean yahoo has good anti-DDoS measures at all, my personal blog site might have better anti-DDoS measures, but since it's small, it's easier to attack.
The fact that ZERO PoS systems have been attacked, even though many of them are tiny, speaks volumes about PoS security.
ALL of your attack vectors remains a theory at best. If you want to prove your point, the best method is not theorycraft further, but actually go and attack one currently public and working PoS system, you can even pick a tiny one if you wish.
I don't want to get into another discussion with you about Bitshares, since it's pointless to discuss Bitshares with your vivid imagination. You are calling a community voted and approved change by the developer team an "attack", that's just too funny. Can I call Gavin's "block size" increase of 50% per year an attack? I didn't even get to vote on it. I would have preferred another way of handling the block size, damn I'm now alienated and upset!
Btw, Bitcoin with PoW is currently and will always be inflationary at least 10% annually, it is much more inflationary than Bitshares. Due to the 10% PoW mining tax. Bitcoin value will rise only with constant inflow of new money, otherwise Bitcoin value will naturally decrease by at least 10% annually.