This makes them impossible to attack but it also makes them a sham. They're just centralized systems implemented in an inefficient way that gives the appearance of decentralization.
I couldn't agree more with that statement - a lot of people are being deceived with PoS, which is a great sham
e.
ftfy
Well you don't need to find historical keys (in order to rewrite the history of PoS block chains), when you can make them for nearly 0 cost.
Simply buy and sell on an exchange, and your cost will only be the spread.
Then short the coin, and start attacking.
Obviously this doesn't apply to illiquid meaningless microfloat altcoins. We are talking about whether PoS is viable for a mainstream decentralized coin. Not.
For a centralized coin, then anything works, you don't even need PoS nor PoW (except to fool people with).
max reorg depth in NXT is 720 blocks
Checkpoints are centralization.
For a centralized coin, then anything works, you don't even need PoS nor PoW (except to fool people with).
If we don't have decentralization, then the entire plot has been lost.
Do you need an example? Here you go (remember the Chinese mining cartel allegedly controls 65% of the Bitcoin hashrate):
https://www.reddit.com/r/btc/comments/48nnaw/the_truth_comes_out_core_devs_have_convinced/