Even if someone acquired 51% actually executing the attack *profitably* is not easy. They'd have to exchange their own properly acquired bitcoins with a huge value for something in real world of that value (a huge mansion or a company maybe?) but irreversible in nature even if people figured it out and attempted to use legal means to restore it. I don't think there's any such thing. Maybe a huge amount of physical gold if one can find how to hide it securely but even that is super hard.