Then the network can be attacked by hard forks. New nodes (or nodes temporarily offline) will see a reorg of greater than 720 blocks as valid and be permanently forked from the nodes who were online and saw the reorg as invalid because it was too deep.
True. So, how do new Bitcoin nodes handle this issue?
One doesn't need to buy the coins in any period of time. One could take years to acquire the coins.
Sorry, that was my mistake. I meant, first ( buying 51% of all PoS tokens ) and then ( selling 51% of all PoS within 720 block ) are both ambitious goals each and even more ambitious done together.
The clock starts from the point of the reorg which can be just prior to selling the coins. One doesn't even need to buy coins they only need to buy the history of coins. A private key which at one point had unspent outputs worth x coins but today has no unspent outputs ("zero balance address") has no direct value to the owner but to an attacker it has value in attacking the network.
Interesting point. So, if you find many somebodies who would be willing to sell their historical passphrases...
The attack isn't limited to 720 blocks. It just means the attacker will be unable to re-org all nodes.
True.
New nodes would need to make sure they are on the legit chain.
But well, how do "Bitcoin" nodes? <<< I mean having a client looking like Bitcoin and feeling like Bitcoin does not necessarily mean it is the real Bitcoin network you are on, right?
So, new nodes need to verify anyway no matter if PoW or PoS. Is that correct?
However by making a re-org of longer than 720 blocks (if you are right about a lack of centralization) the network can be forked permanently which is equally disruptive and quite a feat for an attack with no cost or risk.
Still the point that you moved the goal posts is good enough for me. I think you do now see that someone can attack the network without cost or risk which was the point you refuted. You have now moved to how difficult it would be to acquire that stake which is a totally different argument.
I see where your are coming from and tend to agree, if I would not substitute 'difficult' by 'expensive'.
It always comes down to that: costs. And I am with you: the costs for maintaining several PoS chains are significantly smaller than maintaining several PoW chains. However, I doubt 720 block to be sufficiently long enough to do any damage.