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.
Of course not. On the contrary. if you want to do a 51% attack on consensus decisions, you need to own 51% of the market cap. That's MUCH MUCH more of a cost (in bitcoin, it would amount to having to buy up 20 billion $ of coins, but your cornering of the market would make it still much, much more expensive).... in order to destroy the system for which you just paid 20 billion ! No miner is ever going to invest 20 billion in mining equipment, because he will not mine 50% of the stake in block rewards and fees !
Also, the cost of a 51% attack in PoW is just the cost of 51% of the mining rewards, and you don't need to be stake holder. From the outside, you simply cannot attack a PoS system, because it is based upon much harder to fake digital signatures.
Currently, ALL of the proof of work ever delivered to bitcoin has a security level of 90 bits. A simple 256 bit key signature has a bit security level of 128 bits, and only costs a few milliwatts of power and an old laptop (or even a mobile phone).
Cryptographically, PoW is pure BS as a protection. Any digital signature beats it with tens of orders of magnitude in "efficiency" (that is, spent resources versus security obtained).
Your arguments are compelling. I do not possess the Cryptography expertise to dismount any of the technical arguments.
I will say this:
51% attack in PoW does not only cost 51% of mining rewards. It is not immediate, and it costs all the energy and capital that was put onto it before any ROI.
Furthermore if you consider it is possible to hold a 51% hashpower, if we estimate like a 40% ROI for mining (and I think this is too much considering mining alone), according to current market cap it should cost total about 13.007.185.339,7 USD to achieve a 51% Hashrate. the actual loss of revenue from an attack today would be 2.410.118.399,27 USD Bringing the total to about 15.5 B.
For a 51% PoS, the cost would be 21.678.642.232,83.
I'd grant you are correct in your reasoning, although it seems pretty obvious to me that none of these actors has any incentive to attack the network. And the bigger the market and the higher its share of the Hashrate or Stake, the lesser the incentive. It works for both.
On another point, as I have been trying to argue, I think Proof of Work is economically more sound than Proof of Stake. You are effectively being backed by Energy. There is only so little a miner is willing to receive for his coins, whereas for Proof of Stake, it is cost of capital. The fact is, a miner is tied for the long haul. Capital not so much. Either you inflate your coin to secure the network, or you guarantee transaction fees sufficiently high enough to prevent capital flight.
Also, for Proof of Stake as a way to mint coins, you are dependent on utility and speculation for your currency to be worth anything. If it is just for consensus, then this last argument doesn't stand.
Am I somewhat being able to convey any part of the argument?