What are you talking about "they will never allow themselves to be discovered"? I can assure you, they will be discovered once they start reversing transactions. The identity doesn't matter. This particular crypto-currency will crash and burn over night, the attackers will be only hurting themselves since they hold over 51% of the currency.
On the other hand, attacking a PoW currency only need about 10% of the marketcap in cost. How does "hash rate is known" make it any harder to attack?? I don't understand, I would think it only makes it easier to plan the attack. In a PoS currency, the "unknown" part is really a safety mechanism, since the attacker can't easily figure out if he could ever obtain 51%, because he don't know who's holding what, and 51% may never be achieved simply because they are not available for sale.
I don't think you will even get PoS supporters to back you up here. All currencies (including crypto) have weaknesses. If you claim that someone reversed your PoS transaction, how would you prove it? Everyone would call you a scammer. At least with PoW you can see orphaned blocks on hundreds if not thousands of nodes to show what happened. You may or may not get the network to help you, but at least they will watch for bad agents. In fact, major nodes are working diligently to prevent these types of problems.