...and at the first complaint of a successful attack the price drops to near-zero, making the attack unprofitable in the end.
It's interesting to think that the economics required to attack the network successfully are made even worse if the attack succeeds.
I wouldn't be so sure, it depends on how far the knowledge of a successful attack travels. If the word drops like a stone hidden on page 15 of this mornings paper, then you'll likely not see a whole lot of movement of the market.
Conversely if it's front page news then a hard fork, miners recompile or change options to mine the correct chain in an attempt to orphan the attack. Also note that the seller would offer incentives to the operators of the biggest pools, who should already be honor bound to ensuring that such an attack does not succeed. After all that's why we've been paying them the big bucks.
I wouldn't count a price drop as something the miners would just ignore and a near-zero one at that is laughable. It would effectively end their business and if it means mining one chain where the future worth of 25BTC is 0USD and mining another, the choice is clear.