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 Miners have greater incentive than Stakers honestly. And Miners cannot cashout so easily, and are open to all kinds of attacks from users and economic majorities. Miners are committed to the long haul. Whereas Stakers are not. And the only way to incentivize that is incentivizing hoarding, which is not what you want.