Right, the majority decides what is an "attack" in that it decides which chain to use, and the longest chain wins. But that's not the issue. People (or automated systems) can decide on their own what is an attack. They see transactions being reversed after several confirmations and say "the blockchain is compromised, so I won't use it anymore."

Yes with PoS it's harder to remove the attacker. But the attacker also stands to lose far more money than the attack can possibly be worth.