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%.
Even monsterer doesn't claim that collecting historic priv keys is a viable attack vector. It was explained why it isn't. He claims that it's easy to collect enough priv keys for this attack in a short timeframe.
There is no way to objectively distinguish a historic key that is respent from a historic transaction that had spent that historic key. This is a double-spend with two chains arguing about which was first.
The only way to distinguish which was first is either a decentralized objectivity which is the PoW longest-chain-rule, or for PoS a centralized objectivity such as community/developer checkpoints.
Please stop wasting my time with nonsense replies.