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.
Would 1 satoshi really do it though?
For example, I go to a bar and buy a drink for 1 satoshi (somehow) and it turns out that my satoshi is counterfeit. How would anyone find out it was counterfeit, and once they did do you really think that bar would turn it into national bitcoin news? Think about real counterfeit money, we know it exists, but we don't let it affect the value of our currency, we accept all money as if it's good real money. If it turns out to be counterfeit then one person may be screwed and pissed, but it sure as hell doesn't make the news. Not even the local news, much less the national news. 1 satoshi is probably not a problem.
You cannot counterfeit bitcoin. What the closest thing to counterfeiting you can do is to double spend a TX. It is also not possible to spend only 1 satoshi at this time as the smallest TX that the network will accept is I believe 5160 satashi.
There are some double spend transactions that have occurred in the past and the confidence of bitcoin was not destroyed.