Look at web search--it's no harder to type www.bing.com instead of www.google.com (it's actually 2 letters easier), and for most common searches, the results will be exactly the same. "What time does the Super Bowl start". "Facebook login".[1]
And yet, Bing has not significantly dented Google's web search market share despite $billions of investment in technology, advertising, incentive plans, etc.
> If Dropbox accidentally destroyed just one person’s file, he said, it could erode the trust of all its users.
Ha! Dropbox destroys user files against the will of users all the time--it's inherent in the concept of a syncing service, and why we warn each other, "Dropbox is not a backup service."
And if you're thinking of just straight-up data corruption or loss, look at Evernote, which does that on a regular basis yet continues to grow.
There's nothing more to this story than the fundamental threats that face any business in any industry: if you fail to please your customers, you leave the door open for competitors. But, that doesn't mean there is no margin for error. Or even a small margin for error.
[1] http://searchenginewatch.com/article/2051199/Facebook-Login-...