So one attack vector is forging with timestamps artifiically advanced by almost 15 secs,
and having many nodes disagree on whether the new block is valid or not...
and having many nodes disagree on whether the new block is valid or not...
You need coins to forge, a lot of them, that's why it's called Proof-of-Stake
Why would you destroy your own investment if you spent billions of USD to buy up 51% of coins (or 90% of coins when TF is implemented)?No. You need the majority of stake in any point of time past, to forge all the history of the future from that point of time on. So any group who ever had 51% of the currency/votes/etc. can rewrite the history at will. Or any group that can coerce any group at any point of time. There is also no mechanism to keep track which of the forks is the correct one.
This is called NaS (nothing-at-stake) attack, because you risk nothing in the present (you may have sold your coins already) to destroy everybody.
With PoW, you permanently need a knowable amount of hashrate and still can accomplish very little really.