A 51% attack would be disastrous even if they only double spent one satoshi. It's the loss of confidence in the technology that's the harm. Bitcoin relies on people believing in it to be a secure store of value.
It would be disastrous but it would also very likely spawn new chain acceptance rules that make this type of attack at lot more difficult.
See:
http://gavintech.blogspot.ch/2012/05/neutralizing-51-attack.htmlSo it would basically be a one-off. Either bitcoin would recover or a more robust altcoin would take its place. Either way they attacker would fail in destroying the technology.