So to summarize, if a credible attacker announces his/her intention to destroy a POS currency, holders of that currency are likely to sell to the attacker very cheaply due to their belief that the attack will be successful and thus their coins will be worth nothing if they sell them. Very interesting point. In fact, it presents a "prisoner's dilemma" of sorts, where an attacker can be thwarted if the majority is able to resist the pressure, but where it's in the interests of an individual to break consensus and deal with the attacker.