In theory, though, this attack could be attempted with smaller stakes. The mechanism was researched, developed, and tested rigorously over eight years before going https://www.xcritical.com/ live. In proof-of-stake, malicious validators can be actively punished ("slashed") and ejected from the validator set, costing a substantial amount of ETH.

By purchasing additional ether to control 66% rather than 51%, the attacker is effectively buying the ability to do ex post reorgs and finality reversions (i.e. change the past as well as control the future). The only real defenses here are the enormous cost of 66% of the total staked ether, and the option to fall back to...