Actually that's not true at all--and that's the entire point. If the attacker is credible enough (i.e. he is likely to spend as much money as necessary to succeed), people will be willing to sell to him for a fraction of the value of the coin because of their assumption that the coin will soon be worth zero once the attack is successful.
Thats also true for PoW. Lets say china or russia can buy up the farms of bitcoins. They announce that they are going to kill bitcoin, and watch how people sell them cheap coins.
I think that if there was such a credible attacker (I think he might be just theoretical), the prices would start falling as people hear the news and the prices would go as down as the attacker's news are credible, as it actually happened with all the china fud for bitcoin....
I think that basically this just means that nothing is safe as long as actors with large amount of centralized capital exist, but we all already knew that, hehe..
Once again, I agree that it's theoretically possible to destroy a POW coin. It's just more expensive since you have to buy hardware (expensive with long production lead-times and rapid technological development) as opposed to coins (potentially cheap if you can panic the market).