I'm not saying proof-of-work is appropriate in all cases. To use a more extreme value, a vault containing gold that was protected by proof of work would be use

I'm not saying proof-of-work is appropriate in all cases. To use a more extreme value, a vault containing gold that was protected by proof of work would be useless, since the work required to open it would have to equal the value of the contents in order to discourage thieves. My point was that in some cases, like bitcoin, it might be possible that the value to the attacker of a successful attack on bitcoin was significant, but still much less than the total worth of bitcoin.