"They built a huge business with one fucking product. One."
That's not a good sign, that's a bad one. That means if anyone attacks that one product credibly, the whole company is rocked hard. Better to be diversified, broadly speaking. (Details matter, of course.)
What are the odds Google, Amazon, and a not insignificant number of the other backup vendors in the world are planning some form of direct competition?
... actually, this speaks to what sort of concerns me about a bubble right now. Excessive valuations aren't themselves a cause, they have to be a symptom of something else. ISTM there is an excessive optimism over any one company's ability to completely lock up the market forever and ever, amen. To be honest I don't expect one masterstroke that comes in and just destroys Dropbox in three days, but valuating them as if it's just an assured thing that they're going to completely own the home backup market over the next ten years is absurdly optimistic. It's one possible outcome, but not anywhere near the most likely. Valuating Facebook as if it's going to own social networking forever and ever is absurd; look at the very rapid inroads G+ is making, my point being that even if G+ crashes and burns it has clearly demonstrated that it has a chance, and that means other competition has a chance, too. Valuating X as if it's going to own the market is stupid, and a lot of the valuations seem to be based on that; not a reasonable, rational assessment of the possibility that they may own the market or at least do well, but valuations based on the solid, near-100% of that outcome. Seems unwise to me.