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).
So let's say if Bitcoin was PoS, your attack plan is:
step 1: buy up 50% of all available Bitcoin?
uh good luck with that.
It is so tiring to reply to the hordes of ignorant trolls.
I wrote upthread that one could buy and sell the coins on an exchange. They would then hold the historic private keys to attack with. This would only cost them the average spread between buy and sell prices, so they don't actually have to buy 50%.