Your comment makes me wonder if Dropbox has an exit strategy. Or if they're going to do like Digg and sit there until the entire industry moves on. With anything cloud-related being big busines right now, Dropbox should quit winners and either sell-out or go public.
I don't believe Dropbox has an exit strategy, at least not for the time being. From what Drew Houston has to say about the company, the big vision is to give everyone universal access to their files, regardless of where they are or what platform they're on . That's something that they seem willing to commit to for the long haul.
http://youtu.be/eaJKPCdimLA - actual quote being (21:16 mins): 'I'm completely dedicated to Dropbox ... I mean we're starting with files but there's all kinds of other directions we can go ... we're a long long way away from you being able to sit down in front of any computer, device, and have access to all your stuff.'
The analogy that Drew used was Athena, which apparently is this infrastructure at MIT that gives you access to your stuff no matter which workstation you're on, on campus.
In the context of GP comment ("sit until the industry moves on") I think Dropbox has one potential problem -- that people would start realizing that their data is their data and paying more attention to the privacy aspect of the "storing in the cloud" services. They sure are convenient, but the loss of tight control of one's private data is an issue.
It all goes in cycles, and if something happens that prompts companies and individuals to lock down their files, then Dropbox and alike will go from "universal file access" service to a trivial file sharing app. Still useful, but not as grand.
That's a good point. I'd add that such cycles tend to happen within geographical boundaries (e.g.: this year, something happens in France that makes people in Europe lock down; the next year it's the US, etc) and so it's unlikely that Dropbox would grind to a complete halt, seeing as they've become increasingly international.