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 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)
I just feel that if you are looking for less control and a more costly attack, proof of work is better. Also, your reasoning implies that people are rewarded for hoarding stake rather than transact it. This makes sense for company shares. Not for any currency.
As for the insiders killing it, it is irrelevant.
What is relevant is the cost to attack. Because an outsider can just buy enough stake at X cost and become an insider. As for Proof of Work, I believe it is more costly
I think you are misusing the terms here
First, what you mean by "the cost to attack" is actually the cost of buying the controlling stake. If you are willing to buy something and then destroy it, more power to you. But don't speak about an attack here since the notion of attack assumes a hostile action, something which is not desired (in the case of PoW system, by the majority of "system" users). If you are an owner of a company and you want to liquidate it, you can't talk about "attack" there. Further, whether it is more costly or not is in fact a matter of belief since (major) stake holders may simply refuse to sell you their stakes at any price, and then you are stuck, as simple as it gets. This is obviously not the case with PoW. I guess that's where the primary, fundamental difference between these two systems lies. In other words, you can't "attack" the PoS system, you can destroy it only via "voluntary" action