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).