There are concerns (or concern trolls) about an attacker buying old keys for almost nothing, trashing the chain, then profiting by shorting. And there are concerns that the honest majority can't figure out how to switch to the honest fork.
>And there are concerns that the honest majority can't figure out how to switch to the honest fork.
If that's always an option why bother with proof of stake in the first place? Just let it do whatever and switch to the "correct" fork once in a while!
Why not have the Supreme Court resolve every dispute? Because it's too expensive. Ultimately you can't exempt yourself from the judgement of society and the market. If you commit to a "most-work chain wins no matter what" policy it doesn't mean that chain will win economically.
For people like me who haven't heard of this feature: https://docs.cardano.org/projects/cardano-node/en/latest/sta... It's true that this prevents an attacker from using current keys to perform a long-range attack but old keys can still be used for attacks.
