We are discussing the technical soundness of PoS here, not the economical soundness of attacks.
You mean we just skip straight to you possessing 51% of the coins?
rpietila mentioned it's cheaper to attack PoS, that's economical soundness, I was answering to him.
Perhaps you should keep asking on nxtforum.org to get answers, be persistent, if you're interested, don't expect being spoon-fed answers.