-The BTCs owners could be very sad, because panic could trigger a drop in the FRN/BTC exchange ratio, triggering evaporation of purchasing power of their BTCs, e.g. their BTCs can buy much less goods and services than before the attack.
The attacker must hold a large amount of BTC in order to execute the attack. So he'll also be affected by the lower price. If he brings the price of BTC to 0, then his attack was pointless, since the money that he got back is now worthless.
The attacker would have the advantage of knowing an attack had happened before all others knew. And a smart attacker wouldn't just trade his coins for different coins he'd try to get stuff. But as I've been explaining it takes a long time to get the stuff and you have to maintain the pullback ability for a long time, not 1 hour, if you want to get a bank transfer or bullion shipped.