Under the above assumption that the risks presented by both are the same, I predict it is less costly for a Big Stake Holder to attack the system rather than a Big Mining Pool
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I don't see how proof of stake is better security and more decentralization. But then again, I don't feel Bitcoin is centralized when you can fork it anytime you want. It is effectively impossible to control bar physical coercion. Your argument to defend against a centralized Proof of Stake also stands for Proof of Work. Effectively fork it and let them keep their coin
It kinda looks that you are not really looking for an explanation
Stake holders are what the system itself is made up of, so you can't possibly exclude or eliminate the possibility of shutting it down by those who essentially own it. This is a natural course of things or events, you either render the system vulnerable from outside (PoW) or leave it vulnerable from inside (PoS), there is no third option available in this world (provided you go for some level of invulnerability, of course). But with the PoS system only the insiders (read owners) can kill it (but that will be their deliberate choice), while with the PoW one, anyone who has enough power can do that (it is just a matter of resources) and you don't need to become a major stake holder as is the case with the PoS system (the latter would be equal to buying and owning it)